diff --git a/en_US.ISO8859-1/books/handbook/basics/chapter.sgml b/en_US.ISO8859-1/books/handbook/basics/chapter.sgml index 2487e4d360..fc795a377a 100644 --- a/en_US.ISO8859-1/books/handbook/basics/chapter.sgml +++ b/en_US.ISO8859-1/books/handbook/basics/chapter.sgml @@ -1,1799 +1,1699 @@ Chris Shumway Rewritten by Unix Basics Synopsis basics The following chapter will cover the basic commands and functionality of the FreeBSD operating system. Much of this material is relevant for any Unix-like operating system. Feel free to skim over this chapter if you are familiar with the material. If you are new to FreeBSD, then you will definitely want to read through this chapter carefully. After reading this chapter, you will know: How Unix file permissions work. - - What file system ACLs are and how to use them. - What processes, daemons, and signals are. What a shell is, and how to change your default login environment. How to use basic text editors. How to read manual pages for more information. How to use the virtual consoles of FreeBSD. Permissions Unix FreeBSD, being a direct descendant of BSD Unix, is based on several key Unix concepts. The first, and most pronounced, is that FreeBSD is a multi-user operating system. The system can handle several users all working simultaneously on completely unrelated tasks. The system is responsible for properly sharing and managing requests for hardware devices, peripherals, memory, and CPU time evenly to each user. Because the system is capable of supporting multiple users, everything the system manages has a set of permissions governing who can read, write, and execute the resource. These permissions are stored as two octets broken into three pieces, one for the owner of the file, one for the group that the file belongs to, and one for everyone else. This numerical representation works like this: permissions file permissions Value Permission Directory Listing 0 No read, no write, no execute --- 1 No read, no write, execute --x 2 No read, write, no execute -w- 3 No read, write, execute -wx 4 Read, no write, no execute r-- 5 Read, no write, execute r-x 6 Read, write, no execute rw- 7 Read, write, execute rwx ls directories You can use the command line argument to &man.ls.1; to view a long directory listing that includes a column with information about a file's permissions for the owner, group, and everyone else. Here is how the first column of ls -l is broken up: -rw-r--r-- The first (leftmost) character tells if this file is a regular file, a directory, a special character device, a socket, or any other special pseudo-file device. In this case, the - indicates a regular file. The next three characters, rw- in this example, give the permissions for the owner of the file. The next three characters, r--, give the permissions for the group that the file belongs to. The final three characters, r--, give the permissions for the rest of the world. A dash means that the permission is turned off. In the case of this file, the permissions are set so the owner can read and write to the file, the group can read the file, and the rest of the world can only read the file. According to the table above, the permissions for this file would be 644, where each digit represents the three parts of the file's permission. This is all well and good, but how does the system control permissions on devices? FreeBSD actually treats most hardware devices as a file that programs can open, read, and write data to just like any other file. These special device files are stored on the /dev directory. Directories are also treated as files. They have read, write, and execute permissions. The executable bit for a directory has a slightly different meaning than that of files. When a directory is marked executable, it means it can be moved into, i.e. it is possible to cd into it. This also means that within the directory it is possible to access files whose names are known (subject, of course, to the permissions on the files themselves). In particular, in order to perform a directory listing, read permission must be set on the directory, whilst to delete a file that one knows the name of, it is necessary to have write and execute permissions to the directory containing the file. There are more permission bits, but they are primarily used in special circumstances such as setuid binaries and sticky directories. If you want more information on file permissions and how to set them, be sure to look at the &man.chmod.1; manual page. - - - - - Tom - Rhodes - Contributed by - - - - - ACL - - File System Access Control Lists - - In conjunction with file system enhancements like snapshots, FreeBSD 5.0 - and later offers the security of File System Access Control Lists - (ACLs). - - Access Control Lists extend the standard UNIX - permission model in a highly compatible (POSIX.1e) way. This feature - permits an administrator to make use of and take advantage of a - more sophisticated security model. - - To enable ACL support for UFS - file systems, the following: - - options UFS_ACL - - must be compiled into the kernel. If this option has - not been compiled in, a warning message will be displayed - when attempting to mount a file system supporting ACLs. - This option is included in the GENERIC kernel. - ACLs rely on extended attributes being enabled on - the file system. Extended attributes are natively supported in the next generation - UNIX file system, UFS2. - - A higher level of administrative overhead is required to - configure extended attributes on UFS1 than on - UFS2. The performance of extended attributes - on UFS2 is also substantially higher. As a - result, UFS2 is generally recommended in preference - to UFS1 for use with access control lists. - - ACLs are enabled by the mount-time administrative - flag, , which may be added to /etc/fstab. - The mount-time flag can also be automatically set in a persistent manner using - &man.tunefs.8; to modify a superblock ACLs flag in the - file system header. In general, it is preferred to use the superblock flag - for several reasons: - - - - The mount-time ACLs flag cannot be changed by a - remount (&man.mount.8; ), only by means of a complete - &man.umount.8; and fresh &man.mount.8;. This means that - ACLs cannot be enabled on the root file system after boot. - It also means that you cannot change the disposition of a file system once - it is in use. - - - - Setting the superblock flag will cause the file system to always be - mounted with ACLs enabled even if there is not an - fstab entry or if the devices re-order. This prevents - accidental mounting of the file system without ACLs - enabled, which can result in ACLs being improperly enforced, - and hence security problems. - - - - We may change the ACLs behavior to allow the flag to - be enabled without a complete fresh &man.mount.8;, but we consider it desirable to - discourage accidental mounting without ACLs enabled, because you - can shoot your feet quite nastily if you enable ACLs, then disable - them, then re-enable them without flushing the extended attributes. In general, once - you have enabled ACLs on a file system, they should not be disabled, - as the resulting file protections may not be compatible with those intended by the - users of the system, and re-enabling ACLs may re-attach the previous - ACLs to files that have since had their permissions changed, - resulting in other unpredictable behavior. - - File systems with ACLs enabled will show a + - (plus) sign in their permission settings when viewed. For example: - - drwx------ 2 robert robert 512 Dec 27 11:54 private -drwxrwx---+ 2 robert robert 512 Dec 23 10:57 directory1 -drwxrwx---+ 2 robert robert 512 Dec 22 10:20 directory2 -drwxrwx---+ 2 robert robert 512 Dec 27 11:57 directory3 -drwxr-xr-x 2 robert robert 512 Nov 10 11:54 public_html - - Here we see that the directory1, - directory2, and directory3 - directories are all taking advantage of ACLs. The - public_html directory is not. - - Directory Structure directory hierarchy The FreeBSD directory hierarchy is fundamental to obtaining an overall understanding of the system. The most important concept to grasp is that of the root directory, /. This directory is the first one mounted at boot time and it contains the base system necessary to prepare the operating system for multi-user operation. The root directory also contains mount points for every other file system that you may want to mount. A mount point is a directory where additional file systems can be grafted onto the root file system. Standard mount points include /usr, /var, /mnt, and /cdrom. These directories are usually referenced to entries in the file /etc/fstab. /etc/fstab is a table of various file systems and mount points for reference by the system. Most of the file systems in /etc/fstab are mounted automatically at boot time from the script &man.rc.8; unless they contain the option. Consult the &man.fstab.5; manual page for more information on the format of the /etc/fstab file and the options it contains. A complete description of the file system hierarchy is available in &man.hier.7;. For now, a brief overview of the most common directories will suffice. Directory Description / Root directory of the file system. /bin/ User utilities fundamental to both single-user and multi-user environments. /boot/ Programs and configuration files used during operating system bootstrap. /boot/defaults/ Default bootstrapping configuration files; see &man.loader.conf.5;. /dev/ Device nodes; see &man.intro.4;. /etc/ System configuration files and scripts. /etc/defaults/ Default system configuration files; see &man.rc.8;. /etc/mail/ Configuration files for mail transport agents such as &man.sendmail.8;. /etc/namedb/ named configuration files; see &man.named.8;. /etc/periodic/ Scripts that are run daily, weekly, and monthly, via &man.cron.8;; see &man.periodic.8;. /etc/ppp/ ppp configuration files; see &man.ppp.8;. /mnt/ Empty directory commonly used by system administrators as a temporary mount point. /proc/ Process file system; see &man.procfs.5;, &man.mount.procfs.8;. /root/ Home directory for the root account. /sbin/ System programs and administration utilities fundamental to both single-user and multi-user environments. /stand/ Programs used in a standalone environment. /tmp/ Temporary files, usually a &man.mfs.8; memory-based file system (the contents of /tmp are usually NOT preserved across a system reboot). /usr/ The majority of user utilities and applications. /usr/bin/ Common utilities, programming tools, and applications. /usr/include/ Standard C include files. /usr/lib/ Archive libraries. /usr/libdata/ Miscellaneous utility data files. /usr/libexec/ System daemons & system utilities (executed by other programs). /usr/local/ Local executables, libraries, etc. Also used as the default destination for the FreeBSD ports framework. Within /usr/local, the general layout sketched out by &man.hier.7; for /usr should be used. Exceptions are the man directory, which is directly under /usr/local rather than under /usr/local/share, and the ports documentation is in share/doc/port. /usr/obj/ Architecture-specific target tree produced by building the /usr/src tree. /usr/ports The FreeBSD ports collection (optional). /usr/sbin/ System daemons & system utilities (executed by users). /usr/share/ Architecture-independent files. /usr/src/ BSD and/or local source files. /usr/X11R6/ X11R6 distribution executables, libraries, etc (optional). /var/ Multi-purpose log, temporary, transient, and spool files. /var/log/ Miscellaneous system log files. /var/mail/ User mailbox files. /var/spool/ Miscellaneous printer and mail system spooling directories. /var/tmp/ Temporary files that are kept between system reboots. /var/yp NIS maps. Mounting and Unmounting File systems The file system is best visualized as a tree, rooted, as it were, at /. /dev, /usr, and the other directories in the root directory are branches, which may have their own branches, such as /usr/local, and so on. root file system There are various reasons to house some of these directories on separate file systems. /var contains the directories log/, spool/, and various types of temporary files, and as such, may get filled up. Filling up the root file system is not a good idea, so splitting /var from / is often favorable. Another common reason to contain certain directory trees on other file systems is if they are to be housed on separate physical disks, or are separate virtual disks, such as Network File System mounts, or CDROM drives. The <filename>fstab</filename> File file systems mounted with fstab During the boot process, file systems listed in /etc/fstab are automatically mounted (unless they are listed with the option). The /etc/fstab file contains a list of lines of the following format: device /mount-point fstype options dumpfreq passno device A device name (which should exist), as explained in . mount-point A directory (which should exist), on which to mount the file system. fstype The file system type to pass to &man.mount.8;. The default FreeBSD file system is ufs. options Either for read-write file systems, or for read-only file systems, followed by any other options that may be needed. A common option is for file systems not normally mounted during the boot sequence. Other options are listed in the &man.mount.8; manual page. dumpfreq This is used by &man.dump.8; to determine which file systems require dumping. If the field is missing, a value of zero is assumed. passno This determines the order in which file systems should be checked. File systems that should be skipped should have their passno set to zero. The root file system (which needs to be checked before everything else) should have it's passno set to one, and other file systems' passno should be set to values greater than one. If more than one file systems have the same passno then &man.fsck.8; will attempt to check file systems in parallel if possible. The <command>mount</command> Command file systems mounting The &man.mount.8; command is what is ultimately used to mount file systems. In its most basic form, you use: &prompt.root; mount device mountpoint There are plenty of options, as mentioned in the &man.mount.8; manual page, but the most common are: Mount Options Mount all the file systems listed in /etc/fstab. Exceptions are those marked as noauto, excluded by the flag, or those that are already mounted. Do everything except for the actual system call. This option is useful in conjunction with the flag to determine what &man.mount.8; is actually trying to do. Force the mount of an unclean file system (dangerous), or forces the revocation of write access when downgrading a file system's mount status from read-write to read-only. Mount the file system read-only. This is identical to using the argument to the option. fstype Mount the given file system as the given file system type, or mount only file systems of the given type, if given the option. ufs is the default file system type. Update mount options on the file system. Be verbose. Mount the file system read-write. The option takes a comma-separated list of the options, including the following: nodev Do not interpret special devices on the file system. This is a useful security option. noexec Do not allow execution of binaries on this file system. This is also a useful security option. nosuid Do not interpret setuid or setgid flags on the file system. This is also a useful security option. The <command>umount</command> Command file systems unmounting The &man.umount.8; command takes, as a parameter, one of a mountpoint, a device name, or the or option. All forms take to force unmounting, and for verbosity. Be warned that is not generally a good idea. Forcibly unmounting file systems might crash the computer or damage data on the file system. and are used to unmount all mounted file systems, possibly modified by the file system types listed after . , however, does not attempt to unmount the root file system. Processes FreeBSD is a multi-tasking operating system. This means that it seems as though more than one program is running at once. Each program running at any one time is called a process. Every command you run will start at least one new process, and there are a number of system processes that run all the time, keeping the system functional. Each process is uniquely identified by a number called a process ID, or PID, and, like files, each process also has one owner and group. The owner and group information is used to determine what files and devices the process can open, using the file permissions discussed earlier. Most processes also have a parent process. The parent process is the process that started them. For example, if you are typing commands to the shell then the shell is a process, and any commands you run are also processes. Each process you run in this way will have your shell as its parent process. The exception to this is a special process called init. init is always the first process, so its PID is always 1. init is started automatically by the kernel when FreeBSD starts. Two commands are particularly useful to see the processes on the system, &man.ps.1; and &man.top.1;. The &man.ps.1; command is used to show a static list of the currently running processes, and can show their PID, how much memory they are using, the command line they were started with, and so on. The &man.top.1; command displays all the running processes, and updates the display every few seconds, so that you can interactively see what your computer is doing. By default, &man.ps.1; only shows you the commands that are running and are owned by you. For example: &prompt.user; ps PID TT STAT TIME COMMAND 298 p0 Ss 0:01.10 tcsh 7078 p0 S 2:40.88 xemacs mdoc.xsl (xemacs-21.1.14) 37393 p0 I 0:03.11 xemacs freebsd.dsl (xemacs-21.1.14) 48630 p0 S 2:50.89 /usr/local/lib/netscape-linux/navigator-linux-4.77.bi 48730 p0 IW 0:00.00 (dns helper) (navigator-linux-) 72210 p0 R+ 0:00.00 ps 390 p1 Is 0:01.14 tcsh 7059 p2 Is+ 1:36.18 /usr/local/bin/mutt -y 6688 p3 IWs 0:00.00 tcsh 10735 p4 IWs 0:00.00 tcsh 20256 p5 IWs 0:00.00 tcsh 262 v0 IWs 0:00.00 -tcsh (tcsh) 270 v0 IW+ 0:00.00 /bin/sh /usr/X11R6/bin/startx -- -bpp 16 280 v0 IW+ 0:00.00 xinit /home/nik/.xinitrc -- -bpp 16 284 v0 IW 0:00.00 /bin/sh /home/nik/.xinitrc 285 v0 S 0:38.45 /usr/X11R6/bin/sawfish As you can see in this example, the output from &man.ps.1; is organized into a number of columns. PID is the process ID discussed earlier. PIDs are assigned starting from 1, go up to 99999, and wrap around back to the beginning when you run out. TT shows the tty the program is running on, and can safely be ignored for the moment. STAT shows the program's state, and again, can be safely ignored. TIME is the amount of time the program has been running on the CPU—this is not necessarily the elapsed time since you started the program, as some programs spend a lot of time waiting for things to happen before they need to spend time on the CPU. Finally, COMMAND is the command line that was used to run the program. &man.ps.1; supports a number of different options to change the information that is displayed. One of the most useful sets is auxww. displays information about all the running processes, not just your own. displays the username of the process' owner, as well as memory usage. displays information about daemon processes, and causes &man.ps.1; to display the full command line, rather than truncating it once it gets too long to fit on the screen. The output from &man.top.1; is similar. A sample session looks like this: &prompt.user; top last pid: 72257; load averages: 0.13, 0.09, 0.03 up 0+13:38:33 22:39:10 47 processes: 1 running, 46 sleeping CPU states: 12.6% user, 0.0% nice, 7.8% system, 0.0% interrupt, 79.7% idle Mem: 36M Active, 5256K Inact, 13M Wired, 6312K Cache, 15M Buf, 408K Free Swap: 256M Total, 38M Used, 217M Free, 15% Inuse PID USERNAME PRI NICE SIZE RES STATE TIME WCPU CPU COMMAND 72257 nik 28 0 1960K 1044K RUN 0:00 14.86% 1.42% top 7078 nik 2 0 15280K 10960K select 2:54 0.88% 0.88% xemacs-21.1.14 281 nik 2 0 18636K 7112K select 5:36 0.73% 0.73% XF86_SVGA 296 nik 2 0 3240K 1644K select 0:12 0.05% 0.05% xterm 48630 nik 2 0 29816K 9148K select 3:18 0.00% 0.00% navigator-linu 175 root 2 0 924K 252K select 1:41 0.00% 0.00% syslogd 7059 nik 2 0 7260K 4644K poll 1:38 0.00% 0.00% mutt ... The output is split into two sections. The header (the first five lines) shows the PID of the last process to run, the system load averages (which are a measure of how busy the system is), the system uptime (time since the last reboot) and the current time. The other figures in the header relate to how many processes are running (47 in this case), how much memory and swap space has been taken up, and how much time the system is spending in different CPU states. Below that are a series of columns containing similar information to the output from &man.ps.1;. As before you can see the PID, the username, the amount of CPU time taken, and the command that was run. &man.top.1; also defaults to showing you the amount of memory space taken by the process. This is split into two columns, one for total size, and one for resident size—total size is how much memory the application has needed, and the resident size is how much it is actually using at the moment. In this example you can see that Netscape has required almost 30 MB of RAM, but is currently only using 9 MB. &man.top.1; automatically updates this display every two seconds; this can be changed with the option. Daemons, Signals, and Killing Processes When you run an editor it is easy to control the editor, tell it to load files, and so on. You can do this because the editor provides facilities to do so, and because the editor is attached to a terminal. Some programs are not designed to be run with continuous user input, and so they disconnect from the terminal at the first opportunity. For example, a web server spends all day responding to web requests, it normally does not need any input from you. Programs that transport email from site to site are another example of this class of application. We call these programs daemons. Daemons were characters in Greek mythology; neither good or evil, they were little attendant spirits that, by and large, did useful things for mankind. Much like the web servers and mail servers of today do useful things. This is why the BSD mascot has, for a long time, been the cheerful looking daemon with sneakers and a pitchfork. There is a convention to name programs that normally run as daemons with a trailing d. BIND is the Berkeley Internet Name Daemon (and the actual program that executes is called named), the Apache web server program is called httpd, the line printer spooling daemon is lpd and so on. This is a convention, not a hard and fast rule; for example, the main mail daemon for the Sendmail application is called sendmail, and not maild, as you might imagine. Sometimes you will need to communicate with a daemon process. These communications are called signals, and you can communicate with daemons (or with any running process) by sending it a signal. There are a number of different signals that you can send—some of them have a specific meaning, others are interpreted by the application, and the application's documentation will tell you how that application interprets signals. You can only send a signal to a process that you own. If you send a signal to someone else's process with &man.kill.1; or &man.kill.2; permission will be denied. The exception to this is the root user, who can send signals to everyone's processes. FreeBSD will also send applications signals in some cases. If an application is badly written, and tries to access memory that it is not supposed to, FreeBSD sends the process the Segmentation Violation signal (SIGSEGV). If an application has used the &man.alarm.3; system call to be alerted after a period of time has elapsed then it will be sent the Alarm signal (SIGALRM), and so on. Two signals can be used to stop a process, SIGTERM and SIGKILL. SIGTERM is the polite way to kill a process; the process can catch the signal, realize that you want it to shut down, close any log files it may have open, and generally finish whatever it is doing at the time before shutting down. In some cases a process may even ignore SIGTERM if it is in the middle of some task that can not be interrupted. SIGKILL can not be ignored by a process. This is the I do not care what you are doing, stop right now signal. If you send SIGKILL to a process then FreeBSD will stop that process there and then Not quite true—there are a few things that can not be interrupted. For example, if the process is trying to read from a file that is on another computer on the network, and the other computer has gone away for some reason (been turned off, or the network has a fault), then the process is said to be uninterruptible. Eventually the process will time out, typically after two minutes. As soon as this time out occurs the process will be killed. . The other signals you might want to use are SIGHUP, SIGUSR1, and SIGUSR2. These are general purpose signals, and different applications will do different things when they are sent. Suppose that you have changed your web server's configuration file—you would like to tell the web server to re-read its configuration. You could stop and restart httpd, but this would result in a brief outage period on your web server, which may be undesirable. Most daemons are written to respond to the SIGHUP signal by re-reading their configuration file. So instead of killing and restarting httpd you would send it the SIGHUP signal. Because there is no standard way to respond to these signals, different daemons will have different behavior, so be sure and read the documentation for the daemon in question. Signals are sent using the &man.kill.1; command, as this example shows. Sending a Signal to a Process This example shows how to send a signal to &man.inetd.8;. The &man.inetd.8; configuration file is /etc/inetd.conf, and &man.inetd.8; will re-read this configuration file when it is sent SIGHUP. Find the process ID of the process you want to send the signal to. Do this using &man.ps.1; and &man.grep.1;. The &man.grep.1; command is used to search through output, looking for the string you specify. This command is run as a normal user, and &man.inetd.8; is run as root, so the options must be given to &man.ps.1;. &prompt.user; ps -ax | grep inetd 198 ?? IWs 0:00.00 inetd -wW So the &man.inetd.8; PID is 198. In some cases the grep inetd command might also occur in this output. This is because of the way &man.ps.1; has to find the list of running processes. Use &man.kill.1; to send the signal. Because &man.inetd.8; is being run by root you must use &man.su.1; to become root first. &prompt.user; su Password: &prompt.root; /bin/kill -s HUP 198 In common most with Unix commands, &man.kill.1; will not print any output if it is successful. If you send a signal to a process that you do not own then you will see kill: PID: Operation not permitted. If you mistype the PID you will either send the signal to the wrong process, which could be bad, or, if you are lucky, you will have sent the signal to a PID that is not currently in use, and you will see kill: PID: No such process. Why Use <command>/bin/kill</command>? Many shells provide the kill command as a built in command; that is, the shell will send the signal directly, rather than running /bin/kill. This can be very useful, but different shells have a different syntax for specifying the name of the signal to send. Rather than try to learn all of them, it can be simpler just to use the /bin/kill ... command directly. Sending other signals is very similar, just substitute TERM or KILL in the command line as necessary. Killing random process on the system can be a bad idea. In particular, &man.init.8;, process ID 1, is very special. Running /bin/kill -s KILL 1 is a quick way to shutdown your system. Always double check the arguments you run &man.kill.1; with before you press Return. Shells shells command line In FreeBSD, a lot of everyday work is done in a command line interface called a shell. A shell's main job is to take commands from the input channel and execute them. A lot of shells also have built in functions to help everyday tasks such as file management, file globbing, command line editing, command macros, and environment variables. FreeBSD comes with a set of shells, such as sh, the Bourne Shell, and tcsh, the improved C-shell. Many other shells are available from the FreeBSD Ports Collection, such as zsh and bash. Which shell do you use? It is really a matter of taste. If you are a C programmer you might feel more comfortable with a C-like shell such as tcsh. If you have come from Linux or are new to a Unix command line interface you might try bash. The point is that each shell has unique properties that may or may not work with your preferred working environment, and that you have a choice of what shell to use. One common feature in a shell is filename completion. Given the typing of the first few letters of a command or filename, you can usually have the shell automatically complete the rest of the command or filename by hitting the Tab key on the keyboard. Here is an example. Suppose you have two files called foobar and foo.bar. You want to delete foo.bar. So what you would type on the keyboard is: rm fo[Tab].[Tab]. The shell would print out rm foo[BEEP].bar. The [BEEP] is the console bell, which is the shell telling me it was unable to totally complete the filename because there is more than one match. Both foobar and foo.bar start with fo, but it was able to complete to foo. If you type in ., then hit Tab again, the shell would be able to fill in the rest of the filename for you. environment variables Another feature of the shell is the use of environment variables. Environment variables are a variable key pair stored in the shell's environment space. This space can be read by any program invoked by the shell, and thus contains a lot of program configuration. Here is a list of common environment variables and what they mean: environment variables Variable Description USER Current logged in user's name. PATH Colon separated list of directories to search for binaries. DISPLAY Network name of the X11 display to connect to, if available. SHELL The current shell. TERM The name of the user's terminal. Used to determine the capabilities of the terminal. TERMCAP Database entry of the terminal escape codes to perform various terminal functions. OSTYPE Type of operating system. e.g., FreeBSD. MACHTYPE The CPU architecture that the system is running on. EDITOR The user's preferred text editor. PAGER The user's preferred text pager. MANPATH Colon separated list of directories to search for manual pages. Bourne shells To set an environment variable differs somewhat from shell to shell. For example, in the C-Style shells such as tcsh and csh, you would use setenv to set environment variables. Under Bourne shells such as sh and bash, you would use export to set your current environment variables. For example, to set or modify the EDITOR environment variable, under csh or tcsh a command like this would set EDITOR to /usr/local/bin/emacs: &prompt.user; setenv EDITOR /usr/local/bin/emacs Under Bourne shells: &prompt.user; export EDITOR="/usr/local/bin/emacs" You can also make most shells expand the environment variable by placing a $ character in front of it on the command line. For example, echo $TERM would print out whatever $TERM is set to, because the shell expands $TERM and passes it on to echo. Shells treat a lot of special characters, called meta-characters as special representations of data. The most common one is the * character, which represents any number of characters in a filename. These special meta-characters can be used to do filename globbing. For example, typing in echo * is almost the same as typing in ls because the shell takes all the files that match * and puts them on the command line for echo to see. To prevent the shell from interpreting these special characters, they can be escaped from the shell by putting a backslash (\) character in front of them. echo $TERM prints whatever your terminal is set to. echo \$TERM prints $TERM as is. Changing Your Shell The easiest way to change your shell is to use the chsh command. Running chsh will place you into the editor that is in your EDITOR environment variable; if it is not set, you will be placed in vi. Change the Shell: line accordingly. You can also give chsh the option; this will set your shell for you, without requiring you to enter an editor. For example, if you wanted to change your shell to bash, the following should do the trick: &prompt.user; chsh -s /usr/local/bin/bash Running chsh with no parameters and editing the shell from there would work also. The shell that you wish to use must be present in the /etc/shells file. If you have installed a shell from the ports collection, then this should have been done for you already. If you installed the shell by hand, you must do this. For example, if you installed bash by hand and placed it into /usr/local/bin, you would want to: &prompt.root; echo "/usr/local/bin/bash" >> /etc/shells Then rerun chsh. Text Editors text editors editors A lot of configuration in FreeBSD is done by editing text files. Because of this, it would be a good idea to become familiar with a text editor. FreeBSD comes with a few as part of the base system, and many more are available in the ports collection. ee The easiest and simplest editor to learn is an editor called ee, which stands for easy editor. To start ee, one would type at the command line ee filename where filename is the name of the file to be edited. For example, to edit /etc/rc.conf, type in ee /etc/rc.conf. Once inside of ee, all of the commands for manipulating the editor's functions are listed at the top of the display. The caret ^ character means the Ctrl key on the keyboard, so ^e expands to the key combination Ctrle. To leave ee, hit the Esc key, then choose leave editor. The editor will prompt you to save any changes if the file has been modified. vi editors vi emacs editors emacs FreeBSD also comes with more powerful text editors such as vi as part of the base system, while other editors, like emacs and vim, are part of the FreeBSD Ports Collection. These editors offer much more functionality and power at the expense of being a little more complicated to learn. However if you plan on doing a lot of text editing, learning a more powerful editor such as vim or emacs will save you much more time in the long run. Devices and Device Nodes A device is a term used mostly for hardware-related activities in a system, including disks, printers, graphics cards, and keyboards. When FreeBSD boots, the majority of what FreeBSD displays are devices being detected. You can look through the boot messages again by viewing /var/run/dmesg.boot. For example, acd0 is the first IDE CDROM drive, while kbd0 represents the keyboard. Most of these devices in a Unix operating system must be accessed through special files called device nodes, which are located in the /dev directory. Creating Device Nodes When adding a new device to your system, or compiling in support for additional devices, you may need to create one or more device nodes for the new devices. MAKEDEV Script On systems without DEVFS (this concerns all FreeBSD versions before 5.0), device nodes are created using the &man.MAKEDEV.8; script as shown below: &prompt.root; cd /dev &prompt.root; sh MAKEDEV ad1 This example would make the proper device nodes for the second IDE drive when installed. <literal>DEVFS</literal> (DEVice File System) The device file system, or DEVFS, provides access to kernel's device namespace in the global file system namespace. Instead of having to create and modify device nodes, DEVFS maintains this particular file system for you. See the &man.devfs.5; manual page for more information. DEVFS is used by default in FreeBSD 5.0. Virtual consoles & terminals virtual consoles terminals FreeBSD can be used in various ways. One of them is typing commands to a text terminal. A lot of the flexibility and power of a &unix; operating system is readily available at your hands when using FreeBSD this way. This section describes what terminals and consoles are, and how you can use them in FreeBSD. The console console If you have not configured FreeBSD to automatically start a graphical environment during startup, the system will present you with a login prompt after it boots, right after the startup scripts finish running. You will see something similar to: Additional ABI support:. Local package initialization:. Additional TCP options:. Fri Sep 20 13:01:06 EEST 2002 FreeBSD/i386 (pc3.example.org) (ttyv0) login: The messages might be a bit different on your system, but you will see something similar. The last two lines are what we are interested in right now. The second last line reads: FreeBSD/i386 (pc3.example.org) (ttyv0) This line contains some bits of information about the system you have just booted. You are looking at a FreeBSD console, running on an Intel or compatible processor of the x86 architecture This is what i386 means. Note that even if you are not running FreeBSD on an Intel 386 CPU, this is going to be i386. It is not the type of your processor, but the processor architecture that is shown here. . The name of this machine (every &unix; machine has a name) is pc3.example.org, and you are now looking at its system console—the ttyv0 terminal. Finally, the last line is always: login: This is the part where you are supposed to type in your username to log into FreeBSD. The next section describes how you can do this. Logging into FreeBSD FreeBSD is a multiuser, multiprocessing system. This is the formal description that is usually given to a system that can be used by many different people, who simultaneously run a lot of programs on a single machine. Every multiuser system needs some way to distinguish one user from the rest. In FreeBSD (and all the &unix;-like operating systems), this is accomplished by requiring that every user must log into the system before being able to run programs. Every user has a unique name (the username) and a personal, secret key (the password). FreeBSD will ask for these two before allowing a user to run any programs. startup scripts Right after FreeBSD boots and finishes running its startup scripts Startup scripts are programs that are run automatically by FreeBSD when booting. Their main function is to set things up for everything else to run, and start any services that you have configured to run in the background doing useful things. , it will present you with a prompt and ask for a valid username: login: For the sake of this example, let us assume that your username is john. Type john at this prompt and press Enter. You should then be presented with a prompt to enter a password: login: john Password: Type in john's password now, and press Enter. The password is not echoed! You need not worry about this right now. Suffice it to say that it is done for security reasons. If you have typed your password correctly, you should by now be logged into FreeBSD and ready to try out all the available commands. Multiple consoles Running &unix; commands in one console is fine, but FreeBSD can run many programs at once. Having one console where commands can be typed would be a bit of a waste when an operating system like FreeBSD can run dozens of programs at the same time. This is where virtual consoles can be very helpful. FreeBSD can be configured to present you with many different virtual consoles. You can switch from one of them to any other virtual console by pressing a couple of keys on your keyboard. Each console has its own different output channel, and FreeBSD takes care of properly redirecting keyboard input and monitor output as you switch from one virtual console to the next. Special key combinations have been reserved by FreeBSD for switching consoles A fairly technical and accurate description of all the details of the FreeBSD console and keyboard drivers can be found in the manual pages of &man.syscons.4;, &man.atkbd.4;, &man.vidcontrol.1; and &man.kbdcontrol.1;. We will not expand on the details here, but the interested reader can always consult the manual pages for a more detailed and thorough explanation of how things work. . You can use AltF1, AltF2, through AltF8 to switch to a different virtual console in FreeBSD. As you are switching from one console to the next, FreeBSD takes care of saving and restoring the screen output. The result is an illusion of having multiple virtual screens and keyboards that you can use to type commands for FreeBSD to run. The programs that you launch on one virtual console do not stop running when that console is not visible. They continue running when you have switched to a different virtual console. The <filename>/etc/ttys</filename> file The default configuration of FreeBSD will start up with 8 virtual consoles. This is not a hardwired setting though, and you can easily customize your installation to boot with more or fewer virtual consoles. The number and settings of the virtual consoles are configured in the /etc/ttys file. You can use the /etc/ttys file to configure the virtual consoles of FreeBSD. Each uncommented line in this file (lines that do not start with a # character) contains settings for a single terminal or virtual console. The default version of this file that ships with FreeBSD configures 9 virtual consoles, and enables 8 of them. They are the lines that start with ttyv: # name getty type status comments # ttyv0 "/usr/libexec/getty Pc" cons25 on secure # Virtual terminals ttyv1 "/usr/libexec/getty Pc" cons25 on secure ttyv2 "/usr/libexec/getty Pc" cons25 on secure ttyv3 "/usr/libexec/getty Pc" cons25 on secure ttyv4 "/usr/libexec/getty Pc" cons25 on secure ttyv5 "/usr/libexec/getty Pc" cons25 on secure ttyv6 "/usr/libexec/getty Pc" cons25 on secure ttyv7 "/usr/libexec/getty Pc" cons25 on secure ttyv8 "/usr/X11R6/bin/xdm -nodaemon" xterm off secure For a detailed description of every column in this file and all the options you can use to set things up for the virtual consoles, consult the &man.ttys.5; manual page. Single user mode console A detailed description of what single user mode is can be found in . It is worth noting that there is only one console when you are running FreeBSD in single user mode. There are no virtual consoles available. The settings of the single user mode console can also be found in the /etc/ttys file. Look for the line that starts with console: # name getty type status comments # # If console is marked "insecure", then init will ask for the root password # when going to single-user mode. console none unknown off secure As the comments above the console line indicate, you can edit this line and change secure to insecure. If you do that, when FreeBSD boots into single user mode, it will still ask for the root password. Be careful when changing this to insecure though. If you ever forget the root password, booting into single user mode is a bit involved. It is still possible, but it might be a bit hard for someone who is not very comfortable with the FreeBSD booting process and the programs involved. For More Information Manual Pages manual pages The most comprehensive documentation on FreeBSD is in the form of manual pages. Nearly every program on the system comes with a short reference manual explaining the basic operation and various arguments. These manuals can be viewed with the man command. Use of the man command is simple: &prompt.user; man command command is the name of the command you wish to learn about. For example, to learn more about ls command type: &prompt.user; man ls The online manual is divided up into numbered sections: User commands. System calls and error numbers. Functions in the C libraries. Device drivers. File formats. Games and other diversions. Miscellaneous information. System maintenance and operation commands. Kernel developers. In some cases, the same topic may appear in more than one section of the online manual. For example, there is a chmod user command and a chmod() system call. In this case, you can tell the man command which one you want by specifying the section: &prompt.user; man 1 chmod This will display the manual page for the user command chmod. References to a particular section of the online manual are traditionally placed in parenthesis in written documentation, so &man.chmod.1; refers to the chmod user command and &man.chmod.2; refers to the system call. This is fine if you know the name of the command and simply wish to know how to use it, but what if you cannot recall the command name? You can use man to search for keywords in the command descriptions by using the switch: &prompt.user; man -k mail With this command you will be presented with a list of commands that have the keyword mail in their descriptions. This is actually functionally equivalent to using the apropos command. So, you are looking at all those fancy commands in /usr/bin but do not have the faintest idea what most of them actually do? Simply do: &prompt.user; cd /usr/bin &prompt.user; man -f * or &prompt.user; cd /usr/bin &prompt.user; whatis * which does the same thing. GNU Info Files Free Software Foundation FreeBSD includes many applications and utilities produced by the Free Software Foundation (FSF). In addition to manual pages, these programs come with more extensive hypertext documents called info files which can be viewed with the info command or, if you installed emacs, the info mode of emacs. To use the &man.info.1; command, simply type: &prompt.user; info For a brief introduction, type h. For a quick command reference, type ?. diff --git a/en_US.ISO8859-1/books/handbook/security/chapter.sgml b/en_US.ISO8859-1/books/handbook/security/chapter.sgml index e4efdd173b..d7ecbaf340 100644 --- a/en_US.ISO8859-1/books/handbook/security/chapter.sgml +++ b/en_US.ISO8859-1/books/handbook/security/chapter.sgml @@ -1,3857 +1,3958 @@ Matthew Dillon Much of this chapter has been taken from the security(7) manual page by Security security Synopsis This chapter will provide a basic introduction to system security concepts, some general good rules of thumb, and some advanced topics under FreeBSD. A lot of the topics covered here can be applied to system and Internet security in general as well. The Internet is no longer a friendly place in which everyone wants to be your kind neighbor. Securing your system is imperative to protect your data, intellectual property, time, and much more from the hands of hackers and the like. FreeBSD provides an array of utilities and mechanisms to ensure the integrity and security of your system and network. After reading this chapter, you will know: Basic system security concepts, in respect to FreeBSD. About the various crypt mechanisms available in FreeBSD, such as DES and MD5. How to setup one-time password authentication. How to setup Kerberos, another alternative authentication system. How to create firewalls using IPFW. How to configure IPsec. How to configure and use OpenSSH, FreeBSD's SSH implementation. How to configure and load access control extension modules using the TrustedBSD MAC Framework. + + + What file system ACLs are and how to use them. + + Before reading this chapter, you should: Understand basic FreeBSD and Internet concepts. Introduction Security is a function that begins and ends with the system administrator. While all BSD Unix multi-user systems have some inherent security, the job of building and maintaining additional security mechanisms to keep those users honest is probably one of the single largest undertakings of the sysadmin. Machines are only as secure as you make them, and security concerns are ever competing with the human necessity for convenience. Unix systems, in general, are capable of running a huge number of simultaneous processes and many of these processes operate as servers – meaning that external entities can connect and talk to them. As yesterday's mini-computers and mainframes become today's desktops, and as computers become networked and internetworked, security becomes an even bigger issue. Security is best implemented through a layered onion approach. In a nutshell, what you want to do is to create as many layers of security as are convenient and then carefully monitor the system for intrusions. You do not want to overbuild your security or you will interfere with the detection side, and detection is one of the single most important aspects of any security mechanism. For example, it makes little sense to set the schg flags (see &man.chflags.1;) on every system binary because while this may temporarily protect the binaries, it prevents an attacker who has broken in from making an easily detectable change that may result in your security mechanisms not detecting the attacker at all. System security also pertains to dealing with various forms of attack, including attacks that attempt to crash, or otherwise make a system unusable, but do not attempt to compromise the root account (break root). Security concerns can be split up into several categories: Denial of service attacks. User account compromises. Root compromise through accessible servers. Root compromise via user accounts. Backdoor creation. DoS attacks Denial of Service (DoS) security DoS attacks Denial of Service (DoS) Denial of Service (DoS) A denial of service attack is an action that deprives the machine of needed resources. Typically, DoS attacks are brute-force mechanisms that attempt to crash or otherwise make a machine unusable by overwhelming its servers or network stack. Some DoS attacks try to take advantage of bugs in the networking stack to crash a machine with a single packet. The latter can only be fixed by applying a bug fix to the kernel. Attacks on servers can often be fixed by properly specifying options to limit the load the servers incur on the system under adverse conditions. Brute-force network attacks are harder to deal with. A spoofed-packet attack, for example, is nearly impossible to stop, short of cutting your system off from the Internet. It may not be able to take your machine down, but it can saturate your Internet connection. security account compromises A user account compromise is even more common than a DoS attack. Many sysadmins still run standard telnetd, rlogind, rshd, and ftpd servers on their machines. These servers, by default, do not operate over encrypted connections. The result is that if you have any moderate-sized user base, one or more of your users logging into your system from a remote location (which is the most common and convenient way to login to a system) will have his or her password sniffed. The attentive system admin will analyze his remote access logs looking for suspicious source addresses even for successful logins. One must always assume that once an attacker has access to a user account, the attacker can break root. However, the reality is that in a well secured and maintained system, access to a user account does not necessarily give the attacker access to root. The distinction is important because without access to root the attacker cannot generally hide his tracks and may, at best, be able to do nothing more than mess with the user's files, or crash the machine. User account compromises are very common because users tend not to take the precautions that sysadmins take. security backdoors System administrators must keep in mind that there are potentially many ways to break root on a machine. The attacker may know the root password, the attacker may find a bug in a root-run server and be able to break root over a network connection to that server, or the attacker may know of a bug in a suid-root program that allows the attacker to break root once he has broken into a user's account. If an attacker has found a way to break root on a machine, the attacker may not have a need to install a backdoor. Many of the root holes found and closed to date involve a considerable amount of work by the attacker to cleanup after himself, so most attackers install backdoors. A backdoor provides the attacker with a way to easily regain root access to the system, but it also gives the smart system administrator a convenient way to detect the intrusion. Making it impossible for an attacker to install a backdoor may actually be detrimental to your security, because it will not close off the hole the attacker found to break in the first place. Security remedies should always be implemented with a multi-layered onion peel approach and can be categorized as follows: Securing root and staff accounts. Securing root – root-run servers and suid/sgid binaries. Securing user accounts. Securing the password file. Securing the kernel core, raw devices, and filesystems. Quick detection of inappropriate changes made to the system. Paranoia. The next section of this chapter will cover the above bullet items in greater depth. Securing FreeBSD security securing FreeBSD Command vs. Protocol Throughout this document, we will use bold text to refer to a command or application. This is used for instances such as ssh, since it is a protocol as well as command. The sections that follow will cover the methods of securing your FreeBSD system that were mentioned in the last section of this chapter. Securing the <username>root</username> Account and Staff Accounts su First off, do not bother securing staff accounts if you have not secured the root account. Most systems have a password assigned to the root account. The first thing you do is assume that the password is always compromised. This does not mean that you should remove the password. The password is almost always necessary for console access to the machine. What it does mean is that you should not make it possible to use the password outside of the console or possibly even with the &man.su.1; command. For example, make sure that your pty's are specified as being insecure in the /etc/ttys file so that direct root logins via telnet or rlogin are disallowed. If using other login services such as sshd, make sure that direct root logins are disabled there as well. You can do this by editing your /etc/ssh/sshd_config file, and making sure that PermitRootLogin is set to NO. Consider every access method – services such as FTP often fall through the cracks. Direct root logins should only be allowed via the system console. wheel Of course, as a sysadmin you have to be able to get to root, so we open up a few holes. But we make sure these holes require additional password verification to operate. One way to make root accessible is to add appropriate staff accounts to the wheel group (in /etc/group). The staff members placed in the wheel group are allowed to su to root. You should never give staff members native wheel access by putting them in the wheel group in their password entry. Staff accounts should be placed in a staff group, and then added to the wheel group via the /etc/group file. Only those staff members who actually need to have root access should be placed in the wheel group. It is also possible, when using an authentication method such as Kerberos, to use Kerberos' .k5login file in the root account to allow a &man.ksu.1; to root without having to place anyone at all in the wheel group. This may be the better solution since the wheel mechanism still allows an intruder to break root if the intruder has gotten hold of your password file and can break into a staff account. While having the wheel mechanism is better than having nothing at all, it is not necessarily the safest option. An indirect way to secure staff accounts, and ultimately root access is to use an alternative login access method and do what is known as starring out the encrypted password for the staff accounts. Using the &man.vipw.8; command, one can replace each instance of an encrypted password with a single * character. This command will update the /etc/master.passwd file and user/password database to disable password-authenticated logins. A staff account entry such as: foobar:R9DT/Fa1/LV9U:1000:1000::0:0:Foo Bar:/home/foobar:/usr/local/bin/tcsh Should be changed to this: foobar:*:1000:1000::0:0:Foo Bar:/home/foobar:/usr/local/bin/tcsh This change will prevent normal logins from occurring, since the encrypted password will never match *. With this done, staff members must use another mechanism to authenticate themselves such as &man.kerberos.1; or &man.ssh.1; using a public/private key pair. When using something like Kerberos, one generally must secure the machines which run the Kerberos servers and your desktop workstation. When using a public/private key pair with ssh, one must generally secure the machine used to login from (typically one's workstation). An additional layer of protection can be added to the key pair by password protecting the key pair when creating it with &man.ssh-keygen.1;. Being able to star out the passwords for staff accounts also guarantees that staff members can only login through secure access methods that you have setup. This forces all staff members to use secure, encrypted connections for all of their sessions, which closes an important hole used by many intruders: sniffing the network from an unrelated, less secure machine. The more indirect security mechanisms also assume that you are logging in from a more restrictive server to a less restrictive server. For example, if your main box is running all sorts of servers, your workstation should not be running any. In order for your workstation to be reasonably secure you should run as few servers as possible, up to and including no servers at all, and you should run a password-protected screen blanker. Of course, given physical access to a workstation an attacker can break any sort of security you put on it. This is definitely a problem that you should consider, but you should also consider the fact that the vast majority of break-ins occur remotely, over a network, from people who do not have physical access to your workstation or servers. Kerberos Using something like Kerberos also gives you the ability to disable or change the password for a staff account in one place, and have it immediately effect all the machines on which the staff member may have an account. If a staff member's account gets compromised, the ability to instantly change his password on all machines should not be underrated. With discrete passwords, changing a password on N machines can be a mess. You can also impose re-passwording restrictions with Kerberos: not only can a Kerberos ticket be made to timeout after a while, but the Kerberos system can require that the user choose a new password after a certain period of time (say, once a month). Securing Root-run Servers and SUID/SGID Binaries ntalk comsat finger sandboxes sshd telnetd rshd rlogind The prudent sysadmin only runs the servers he needs to, no more, no less. Be aware that third party servers are often the most bug-prone. For example, running an old version of imapd or popper is like giving a universal root ticket out to the entire world. Never run a server that you have not checked out carefully. Many servers do not need to be run as root. For example, the ntalk, comsat, and finger daemons can be run in special user sandboxes. A sandbox is not perfect, unless you go through a large amount of trouble, but the onion approach to security still stands: If someone is able to break in through a server running in a sandbox, they still have to break out of the sandbox. The more layers the attacker must break through, the lower the likelihood of his success. Root holes have historically been found in virtually every server ever run as root, including basic system servers. If you are running a machine through which people only login via sshd and never login via telnetd or rshd or rlogind, then turn off those services! FreeBSD now defaults to running ntalkd, comsat, and finger in a sandbox. Another program which may be a candidate for running in a sandbox is &man.named.8;. /etc/defaults/rc.conf includes the arguments necessary to run named in a sandbox in a commented-out form. Depending on whether you are installing a new system or upgrading an existing system, the special user accounts used by these sandboxes may not be installed. The prudent sysadmin would research and implement sandboxes for servers whenever possible. sendmail There are a number of other servers that typically do not run in sandboxes: sendmail, popper, imapd, ftpd, and others. There are alternatives to some of these, but installing them may require more work than you are willing to perform (the convenience factor strikes again). You may have to run these servers as root and rely on other mechanisms to detect break-ins that might occur through them. The other big potential root holes in a system are the suid-root and sgid binaries installed on the system. Most of these binaries, such as rlogin, reside in /bin, /sbin, /usr/bin, or /usr/sbin. While nothing is 100% safe, the system-default suid and sgid binaries can be considered reasonably safe. Still, root holes are occasionally found in these binaries. A root hole was found in Xlib in 1998 that made xterm (which is typically suid) vulnerable. It is better to be safe than sorry and the prudent sysadmin will restrict suid binaries, that only staff should run, to a special group that only staff can access, and get rid of (chmod 000) any suid binaries that nobody uses. A server with no display generally does not need an xterm binary. Sgid binaries can be almost as dangerous. If an intruder can break an sgid-kmem binary, the intruder might be able to read /dev/kmem and thus read the encrypted password file, potentially compromising any passworded account. Alternatively an intruder who breaks group kmem can monitor keystrokes sent through pty's, including pty's used by users who login through secure methods. An intruder that breaks the tty group can write to almost any user's tty. If a user is running a terminal program or emulator with a keyboard-simulation feature, the intruder can potentially generate a data stream that causes the user's terminal to echo a command, which is then run as that user. Securing User Accounts User accounts are usually the most difficult to secure. While you can impose Draconian access restrictions on your staff and star out their passwords, you may not be able to do so with any general user accounts you might have. If you do have sufficient control, then you may win out and be able to secure the user accounts properly. If not, you simply have to be more vigilant in your monitoring of those accounts. Use of ssh and Kerberos for user accounts is more problematic, due to the extra administration and technical support required, but still a very good solution compared to a crypted password file. Securing the Password File The only sure fire way is to * out as many passwords as you can and use ssh or Kerberos for access to those accounts. Even though the encrypted password file (/etc/spwd.db) can only be read by root, it may be possible for an intruder to obtain read access to that file even if the attacker cannot obtain root-write access. Your security scripts should always check for and report changes to the password file (see the Checking file integrity section below). Securing the Kernel Core, Raw Devices, and Filesystems If an attacker breaks root he can do just about anything, but there are certain conveniences. For example, most modern kernels have a packet sniffing device driver built in. Under FreeBSD it is called the bpf device. An intruder will commonly attempt to run a packet sniffer on a compromised machine. You do not need to give the intruder the capability and most systems do not have the need for the bpf device compiled in. sysctl But even if you turn off the bpf device, you still have /dev/mem and /dev/kmem to worry about. For that matter, the intruder can still write to raw disk devices. Also, there is another kernel feature called the module loader, &man.kldload.8;. An enterprising intruder can use a KLD module to install his own bpf device, or other sniffing device, on a running kernel. To avoid these problems you have to run the kernel at a higher secure level, at least securelevel 1. The securelevel can be set with a sysctl on the kern.securelevel variable. Once you have set the securelevel to 1, write access to raw devices will be denied and special chflags flags, such as schg, will be enforced. You must also ensure that the schg flag is set on critical startup binaries, directories, and script files – everything that gets run up to the point where the securelevel is set. This might be overdoing it, and upgrading the system is much more difficult when you operate at a higher secure level. You may compromise and run the system at a higher secure level but not set the schg flag for every system file and directory under the sun. Another possibility is to simply mount / and /usr read-only. It should be noted that being too Draconian in what you attempt to protect may prevent the all-important detection of an intrusion. Checking File Integrity: Binaries, Configuration Files, Etc. When it comes right down to it, you can only protect your core system configuration and control files so much before the convenience factor rears its ugly head. For example, using chflags to set the schg bit on most of the files in / and /usr is probably counterproductive, because while it may protect the files, it also closes a detection window. The last layer of your security onion is perhaps the most important – detection. The rest of your security is pretty much useless (or, worse, presents you with a false sense of safety) if you cannot detect potential incursions. Half the job of the onion is to slow down the attacker, rather than stop him, in order to give the detection side of the equation a chance to catch him in the act. The best way to detect an incursion is to look for modified, missing, or unexpected files. The best way to look for modified files is from another (often centralized) limited-access system. Writing your security scripts on the extra-secure limited-access system makes them mostly invisible to potential attackers, and this is important. In order to take maximum advantage you generally have to give the limited-access box significant access to the other machines in the business, usually either by doing a read-only NFS export of the other machines to the limited-access box, or by setting up ssh key-pairs to allow the limited-access box to ssh to the other machines. Except for its network traffic, NFS is the least visible method – allowing you to monitor the filesystems on each client box virtually undetected. If your limited-access server is connected to the client boxes through a switch, the NFS method is often the better choice. If your limited-access server is connected to the client boxes through a hub, or through several layers of routing, the NFS method may be too insecure (network-wise) and using ssh may be the better choice even with the audit-trail tracks that ssh lays. Once you give a limited-access box, at least read access to the client systems it is supposed to monitor, you must write scripts to do the actual monitoring. Given an NFS mount, you can write scripts out of simple system utilities such as &man.find.1; and &man.md5.1;. It is best to physically md5 the client-box files at least once a day, and to test control files such as those found in /etc and /usr/local/etc even more often. When mismatches are found, relative to the base md5 information the limited-access machine knows is valid, it should scream at a sysadmin to go check it out. A good security script will also check for inappropriate suid binaries and for new or deleted files on system partitions such as / and /usr. When using ssh rather than NFS, writing the security script is much more difficult. You essentially have to scp the scripts to the client box in order to run them, making them visible, and for safety you also need to scp the binaries (such as find) that those scripts use. The ssh client on the client box may already be compromised. All in all, using ssh may be necessary when running over insecure links, but it is also a lot harder to deal with. A good security script will also check for changes to user and staff members access configuration files: .rhosts, .shosts, .ssh/authorized_keys and so forth… files that might fall outside the purview of the MD5 check. If you have a huge amount of user disk space, it may take too long to run through every file on those partitions. In this case, setting mount flags to disallow suid binaries and devices on those partitions is a good idea. The nodev and nosuid options (see &man.mount.8;) are what you want to look into. You should probably scan them anyway, at least once a week, since the object of this layer is to detect a break-in whether or not the break-in is effective. Process accounting (see &man.accton.8;) is a relatively low-overhead feature of the operating system which might help as a post-break-in evaluation mechanism. It is especially useful in tracking down how an intruder has actually broken into a system, assuming the file is still intact after the break-in occurs. Finally, security scripts should process the log files, and the logs themselves should be generated in as secure a manner as possible – remote syslog can be very useful. An intruder tries to cover his tracks, and log files are critical to the sysadmin trying to track down the time and method of the initial break-in. One way to keep a permanent record of the log files is to run the system console to a serial port and collect the information on a continuing basis through a secure machine monitoring the consoles. Paranoia A little paranoia never hurts. As a rule, a sysadmin can add any number of security features, as long as they do not effect convenience, and can add security features that do effect convenience with some added thought. Even more importantly, a security administrator should mix it up a bit – if you use recommendations such as those given by this document verbatim, you give away your methodologies to the prospective attacker who also has access to this document. Denial of Service Attacks Denial of Service (DoS) This section covers Denial of Service attacks. A DoS attack is typically a packet attack. While there is not much you can do about modern spoofed packet attacks that saturate your network, you can generally limit the damage by ensuring that the attacks cannot take down your servers. Limiting server forks. Limiting springboard attacks (ICMP response attacks, ping broadcast, etc.). Kernel Route Cache. A common DoS attack is against a forking server that attempts to cause the server to eat processes, file descriptors, and memory, until the machine dies. inetd (see &man.inetd.8;) has several options to limit this sort of attack. It should be noted that while it is possible to prevent a machine from going down, it is not generally possible to prevent a service from being disrupted by the attack. Read the inetd manual page carefully and pay specific attention to the , , and options. Note that spoofed-IP attacks will circumvent the option to inetd, so typically a combination of options must be used. Some standalone servers have self-fork-limitation parameters. Sendmail has its option, which tends to work much better than trying to use sendmail's load limiting options due to the load lag. You should specify a MaxDaemonChildren parameter, when you start sendmail, high enough to handle your expected load, but not so high that the computer cannot handle that number of sendmails without falling on its face. It is also prudent to run sendmail in queued mode () and to run the daemon (sendmail -bd) separate from the queue-runs (sendmail -q15m). If you still want real-time delivery you can run the queue at a much lower interval, such as , but be sure to specify a reasonable MaxDaemonChildren option for that sendmail to prevent cascade failures. Syslogd can be attacked directly and it is strongly recommended that you use the option whenever possible, and the option otherwise. You should also be fairly careful with connect-back services such as tcpwrapper's reverse-identd, which can be attacked directly. You generally do not want to use the reverse-ident feature of tcpwrappers for this reason. It is a very good idea to protect internal services from external access by firewalling them off at your border routers. The idea here is to prevent saturation attacks from outside your LAN, not so much to protect internal services from network-based root compromise. Always configure an exclusive firewall, i.e., firewall everything except ports A, B, C, D, and M-Z. This way you can firewall off all of your low ports except for certain specific services such as named (if you are primary for a zone), ntalkd, sendmail, and other Internet-accessible services. If you try to configure the firewall the other way – as an inclusive or permissive firewall, there is a good chance that you will forget to close a couple of services, or that you will add a new internal service and forget to update the firewall. You can still open up the high-numbered port range on the firewall, to allow permissive-like operation, without compromising your low ports. Also take note that FreeBSD allows you to control the range of port numbers used for dynamic binding, via the various net.inet.ip.portrange sysctl's (sysctl -a | fgrep portrange), which can also ease the complexity of your firewall's configuration. For example, you might use a normal first/last range of 4000 to 5000, and a hiport range of 49152 to 65535, then block off everything under 4000 in your firewall (except for certain specific Internet-accessible ports, of course). ICMP_BANDLIM Another common DoS attack is called a springboard attack – to attack a server in a manner that causes the server to generate responses which overloads the server, the local network, or some other machine. The most common attack of this nature is the ICMP ping broadcast attack. The attacker spoofs ping packets sent to your LAN's broadcast address with the source IP address set to the actual machine they wish to attack. If your border routers are not configured to stomp on ping's to broadcast addresses, your LAN winds up generating sufficient responses to the spoofed source address to saturate the victim, especially when the attacker uses the same trick on several dozen broadcast addresses over several dozen different networks at once. Broadcast attacks of over a hundred and twenty megabits have been measured. A second common springboard attack is against the ICMP error reporting system. By constructing packets that generate ICMP error responses, an attacker can saturate a server's incoming network and cause the server to saturate its outgoing network with ICMP responses. This type of attack can also crash the server by running it out of mbuf's, especially if the server cannot drain the ICMP responses it generates fast enough. The FreeBSD kernel has a new kernel compile option called which limits the effectiveness of these sorts of attacks. The last major class of springboard attacks is related to certain internal inetd services such as the udp echo service. An attacker simply spoofs a UDP packet with the source address being server A's echo port, and the destination address being server B's echo port, where server A and B are both on your LAN. The two servers then bounce this one packet back and forth between each other. The attacker can overload both servers and their LANs simply by injecting a few packets in this manner. Similar problems exist with the internal chargen port. A competent sysadmin will turn off all of these inetd-internal test services. Spoofed packet attacks may also be used to overload the kernel route cache. Refer to the net.inet.ip.rtexpire, rtminexpire, and rtmaxcache sysctl parameters. A spoofed packet attack that uses a random source IP will cause the kernel to generate a temporary cached route in the route table, viewable with netstat -rna | fgrep W3. These routes typically timeout in 1600 seconds or so. If the kernel detects that the cached route table has gotten too big it will dynamically reduce the rtexpire but will never decrease it to less than rtminexpire. There are two problems: The kernel does not react quickly enough when a lightly loaded server is suddenly attacked. The rtminexpire is not low enough for the kernel to survive a sustained attack. If your servers are connected to the Internet via a T3 or better, it may be prudent to manually override both rtexpire and rtminexpire via &man.sysctl.8;. Never set either parameter to zero (unless you want to crash the machine). Setting both parameters to 2 seconds should be sufficient to protect the route table from attack. Access Issues with Kerberos and SSH ssh Kerberos There are a few issues with both Kerberos and ssh that need to be addressed if you intend to use them. Kerberos V is an excellent authentication protocol, but there are bugs in the kerberized telnet and rlogin applications that make them unsuitable for dealing with binary streams. Also, by default Kerberos does not encrypt a session unless you use the option. ssh encrypts everything by default. ssh works quite well in every respect except that it forwards encryption keys by default. What this means is that if you have a secure workstation holding keys that give you access to the rest of the system, and you ssh to an insecure machine, your keys are usable. The actual keys themselves are not exposed, but ssh installs a forwarding port for the duration of your login, and if an attacker has broken root on the insecure machine he can utilize that port to use your keys to gain access to any other machine that your keys unlock. We recommend that you use ssh in combination with Kerberos whenever possible for staff logins. ssh can be compiled with Kerberos support. This reduces your reliance on potentially exposable ssh keys while at the same time protecting passwords via Kerberos. ssh keys should only be used for automated tasks from secure machines (something that Kerberos is unsuited to do). We also recommend that you either turn off key-forwarding in the ssh configuration, or that you make use of the from=IP/DOMAIN option that ssh allows in its authorized_keys file to make the key only usable to entities logging in from specific machines. Bill Swingle Parts rewritten and updated by DES, MD5, and Crypt security crypt crypt DES MD5 Every user on a Unix system has a password associated with their account. It seems obvious that these passwords need to be known only to the user and the actual operating system. In order to keep these passwords secret, they are encrypted with what is known as a one-way hash, that is, they can only be easily encrypted but not decrypted. In other words, what we told you a moment ago was obvious is not even true: the operating system itself does not really know the password. It only knows the encrypted form of the password. The only way to get the plain-text password is by a brute force search of the space of possible passwords. Unfortunately the only secure way to encrypt passwords when Unix came into being was based on DES, the Data Encryption Standard. This was not such a problem for users resident in the US, but since the source code for DES could not be exported outside the US, FreeBSD had to find a way to both comply with US law and retain compatibility with all the other Unix variants that still used DES. The solution was to divide up the encryption libraries so that US users could install the DES libraries and use DES but international users still had an encryption method that could be exported abroad. This is how FreeBSD came to use MD5 as its default encryption method. MD5 is believed to be more secure than DES, so installing DES is offered primarily for compatibility reasons. Recognizing Your Crypt Mechanism Before FreeBSD 4.4 libcrypt.a was a symbolic link pointing to the library which was used for encryption. FreeBSD 4.4 changed libcrypt.a to provide a configurable password authentication hash library. Currently the library supports DES, MD5 and Blowfish hash functions. By default FreeBSD uses MD5 to encrypt passwords. It is pretty easy to identify which encryption method FreeBSD is set up to use. Examining the encrypted passwords in the /etc/master.passwd file is one way. Passwords encrypted with the MD5 hash are longer than those encrypted with the DES hash and also begin with the characters $1$. Passwords starting with $2$ are encrypted with the Blowfish hash function. DES password strings do not have any particular identifying characteristics, but they are shorter than MD5 passwords, and are coded in a 64-character alphabet which does not include the $ character, so a relatively short string which does not begin with a dollar sign is very likely a DES password. The password format used for new passwords is controlled by the passwd_format login capability in /etc/login.conf, which takes values of des or md5 or blf. See the &man.login.conf.5; manual page for more information about login capabilities. One-time Passwords One-time Passwords security One-time Passwords S/Key is a one-time password scheme based on a one-way hash function. FreeBSD uses the MD4 hash for compatibility but other systems have used MD5 and DES-MAC. S/Key has been part of the FreeBSD base system since version 1.1.5 and is also used on a growing number of other operating systems. S/Key is a registered trademark of Bell Communications Research, Inc. From version 5.0 of FreeBSD, S/Key has been replaced with the functionally equivalent OPIE (One-time Passwords In Everything). OPIE uses the MD5 hash by default. There are three different sorts of passwords which we will discuss below. The first is your usual Unix-style or Kerberos password; we will call this a Unix password. The second sort is the one-time password which is generated by the S/Key key program or the OPIE opiekey program and accepted by the keyinit or opiepasswd programs and the login prompt; we will call this a one-time password. The final sort of password is the secret password which you give to the key/opiekey programs (and sometimes the keyinit/opiepasswd programs) which it uses to generate one-time passwords; we will call it a secret password or just unqualified password. The secret password does not have anything to do with your Unix password; they can be the same but this is not recommended. S/Key and OPIE secret passwords are not limited to 8 characters like Unix passwords, they can be as long as you like. Passwords of six or seven word long phrases are fairly common. For the most part, the S/Key or OPIE system operates completely independently of the Unix password system. Besides the password, there are two other pieces of data that are important to S/Key and OPIE. One is what is known as the seed or key, consisting of two letters and five digits. The other is what is called the iteration count, a number between 1 and 100. S/Key creates the one-time password by concatenating the seed and the secret password, then applying the MD4/MD5 hash as many times as specified by the iteration count and turning the result into six short English words. These six English words are your one-time password. The authentication system (primarily PAM) keeps track of the last one-time password used, and the user is authenticated if the hash of the user-provided password is equal to the previous password. Because a one-way hash is used it is impossible to generate future one-time passwords if a successfully used password is captured; the iteration count is decremented after each successful login to keep the user and the login program in sync. When the iteration count gets down to 1, S/Key and OPIE must be reinitialized. There are three programs involved in each system which we will discuss below. The key and opiekey programs accept an iteration count, a seed, and a secret password, and generate a one-time password or a consecutive list of one-time passwords. The keyinit and opiepasswd programs are used to initialize S/Key and OPIE respectively, and to change passwords, iteration counts, or seeds; they take either a secret passphrase, or an iteration count, seed, and one-time password. The keyinfo and opieinfo programs examine the relevant credentials files (/etc/skeykeys or /etc/opiekeys) and print out the invoking user's current iteration count and seed. There are four different sorts of operations we will cover. The first is using keyinit or opiepasswd over a secure connection to set up one-time-passwords for the first time, or to change your password or seed. The second operation is using keyinit or opiepasswd over an insecure connection, in conjunction with key or opiekey over a secure connection, to do the same. The third is using key/opiekey to log in over an insecure connection. The fourth is using key or opiekey to generate a number of keys which can be written down or printed out to carry with you when going to some location without secure connections to anywhere. Secure Connection Initialization To initialize S/Key for the first time, change your password, or change your seed while logged in over a secure connection (e.g., on the console of a machine or via ssh), use the keyinit command without any parameters while logged in as yourself: &prompt.user; keyinit Adding unfurl: Reminder - Only use this method if you are directly connected. If you are using telnet or rlogin exit with no password and use keyinit -s. Enter secret password: Again secret password: ID unfurl s/key is 99 to17757 DEFY CLUB PRO NASH LACE SOFT For OPIE, opiepasswd is used instead: &prompt.user; opiepasswd -c [grimreaper] ~ $ opiepasswd -f -c Adding unfurl: Only use this method from the console; NEVER from remote. If you are using telnet, xterm, or a dial-in, type ^C now or exit with no password. Then run opiepasswd without the -c parameter. Using MD5 to compute responses. Enter new secret pass phrase: Again new secret pass phrase: ID unfurl OTP key is 499 to4268 MOS MALL GOAT ARM AVID COED At the Enter new secret pass phrase: or Enter secret password: prompts, you should enter a password or phrase. Remember, this is not the password that you will use to login with, this is used to generate your one-time login keys. The ID line gives the parameters of your particular instance; your login name, the iteration count, and seed. When logging in the system will remember these parameters and present them back to you so you do not have to remember them. The last line gives the particular one-time password which corresponds to those parameters and your secret password; if you were to re-login immediately, this one-time password is the one you would use. Insecure Connection Initialization To initialize or change your secret password over an insecure connection, you will need to already have a secure connection to some place where you can run key or opiekey; this might be in the form of a desk accessory on a Macintosh, or a shell prompt on a machine you trust. You will also need to make up an iteration count (100 is probably a good value), and you may make up your own seed or use a randomly-generated one. Over on the insecure connection (to the machine you are initializing), use the keyinit -s command: &prompt.user; keyinit -s Updating unfurl: Old key: to17758 Reminder you need the 6 English words from the key command. Enter sequence count from 1 to 9999: 100 Enter new key [default to17759]: s/key 100 to 17759 s/key access password: s/key access password:CURE MIKE BANE HIM RACY GORE For OPIE, you need to use opiepasswd: &prompt.user; opiepasswd Updating unfurl: You need the response from an OTP generator. Old secret pass phrase: otp-md5 498 to4268 ext Response: GAME GAG WELT OUT DOWN CHAT New secret pass phrase: otp-md5 499 to4269 Response: LINE PAP MILK NELL BUOY TROY ID mark OTP key is 499 gr4269 LINE PAP MILK NELL BUOY TROY To accept the default seed (which the keyinit program confusingly calls a key), press Return. Then before entering an access password, move over to your secure connection or S/Key desk accessory, and give it the same parameters: &prompt.user; key 100 to17759 Reminder - Do not use this program while logged in via telnet or rlogin. Enter secret password: <secret password> CURE MIKE BANE HIM RACY GORE Or for OPIE: &prompt.user; opiekey 498 to4268 Using the MD5 algorithm to compute response. Reminder: Don't use opiekey from telnet or dial-in sessions. Enter secret pass phrase: GAME GAG WELT OUT DOWN CHAT Now switch back over to the insecure connection, and copy the one-time password generated over to the relevant program. Generating a Single one-time Password Once you have initialized S/Key or OPIE, when you login you will be presented with a prompt like this: &prompt.user; telnet example.com Trying 10.0.0.1... Connected to example.com Escape character is '^]'. FreeBSD/i386 (example.com) (ttypa) login: <username> s/key 97 fw13894 Password: Or for OPIE: &prompt.user; telnet example.com Trying 10.0.0.1... Connected to example.com Escape character is '^]'. FreeBSD/i386 (example.com) (ttypa) login: <username> otp-md5 498 gr4269 ext Password: As a side note, the S/Key and OPIE prompts have a useful feature (not shown here): if you press Return at the password prompt, the prompter will turn echo on, so you can see what you are typing. This can be extremely useful if you are attempting to type in a password by hand, such as from a printout. MS-DOS Windows MacOS At this point you need to generate your one-time password to answer this login prompt. This must be done on a trusted system that you can run key or opiekey on. (There are versions of these for DOS, Windows and MacOS as well.) They need both the iteration count and the seed as command line options. You can cut-and-paste these right from the login prompt on the machine that you are logging in to. On the trusted system: &prompt.user; key 97 fw13894 Reminder - Do not use this program while logged in via telnet or rlogin. Enter secret password: WELD LIP ACTS ENDS ME HAAG For OPIE: &prompt.user; opiekey 498 to4268 Using the MD5 algorithm to compute response. Reminder: Don't use opiekey from telnet or dial-in sessions. Enter secret pass phrase: GAME GAG WELT OUT DOWN CHAT Now that you have your one-time password you can continue logging in: login: <username> s/key 97 fw13894 Password: <return to enable echo> s/key 97 fw13894 Password [echo on]: WELD LIP ACTS ENDS ME HAAG Last login: Tue Mar 21 11:56:41 from 10.0.0.2 ... Generating Multiple one-time Passwords Sometimes you have to go places where you do not have access to a trusted machine or secure connection. In this case, it is possible to use the key and opiekey commands to generate a number of one-time passwords beforehand to be printed out and taken with you. For example: &prompt.user; key -n 5 30 zz99999 Reminder - Do not use this program while logged in via telnet or rlogin. Enter secret password: <secret password> 26: SODA RUDE LEA LIND BUDD SILT 27: JILT SPY DUTY GLOW COWL ROT 28: THEM OW COLA RUNT BONG SCOT 29: COT MASH BARR BRIM NAN FLAG 30: CAN KNEE CAST NAME FOLK BILK Or for OPIE: &prompt.user; opiekey -n 5 30 zz99999 Using the MD5 algorithm to compute response. Reminder: Don't use opiekey from telnet or dial-in sessions. Enter secret pass phrase: <secret password> 26: JOAN BORE FOSS DES NAY QUIT 27: LATE BIAS SLAY FOLK MUCH TRIG 28: SALT TIN ANTI LOON NEAL USE 29: RIO ODIN GO BYE FURY TIC 30: GREW JIVE SAN GIRD BOIL PHI The requests five keys in sequence, the specifies what the last iteration number should be. Note that these are printed out in reverse order of eventual use. If you are really paranoid, you might want to write the results down by hand; otherwise you can cut-and-paste into lpr. Note that each line shows both the iteration count and the one-time password; you may still find it handy to scratch off passwords as you use them. Restricting Use of Unix Passwords S/Key can place restrictions on the use of Unix passwords based on the host name, user name, terminal port, or IP address of a login session. These restrictions can be found in the configuration file /etc/skey.access. The &man.skey.access.5; manual page has more information on the complete format of the file and also details some security cautions to be aware of before depending on this file for security. If there is no /etc/skey.access file (this is the default on FreeBSD 4.x systems), then all users will be allowed to use Unix passwords. If the file exists, however, then all users will be required to use S/Key unless explicitly permitted to do otherwise by configuration statements in the skey.access file. In all cases, Unix passwords are permitted on the console. Here is a sample skey.access configuration file which illustrates the three most common sorts of configuration statements: permit internet 192.168.0.0 255.255.0.0 permit user fnord permit port ttyd0 The first line (permit internet) allows users whose IP source address (which is vulnerable to spoofing) matches the specified value and mask, to use Unix passwords. This should not be considered a security mechanism, but rather, a means to remind authorized users that they are using an insecure network and need to use S/Key for authentication. The second line (permit user) allows the specified username, in this case fnord, to use Unix passwords at any time. Generally speaking, this should only be used for people who are either unable to use the key program, like those with dumb terminals, or those who are uneducable. The third line (permit port) allows all users logging in on the specified terminal line to use Unix passwords; this would be used for dial-ups. OPIE can restrict the use of Unix passwords based on the IP address of a login session just like S/Key does. The relevant file is /etc/opieaccess, which is present by default on FreeBSD 5.0 and newer systems. Please check &man.opieaccess.5; for more information on this file and which security considerations you should be aware of when using it. Here is a sample opieaccess file: permit 192.168.0.0 255.255.0.0 This line allows users whose IP source address (which is vulnerable to spoofing) matches the specified value and mask, to use Unix passwords at any time. If no rules in opieaccess are matched, the default is to deny non-OPIE logins. Mark Murray Contributed by Mark Dapoz Based on a contribution by Kerberos Kerberos Kerberos is a network add-on system/protocol that allows users to authenticate themselves through the services of a secure server. Services such as remote login, remote copy, secure inter-system file copying and other high-risk tasks are made considerably safer and more controllable. The following instructions can be used as a guide on how to set up Kerberos as distributed for FreeBSD. However, you should refer to the relevant manual pages for a complete description. Installing Kerberos MIT Kerberos installing Kerberos is an optional component of FreeBSD. The easiest way to install this software is by selecting the 'krb4' or 'krb5' distribution in sysinstall during the initial installation of FreeBSD. This will install the 'eBones' (KerberosIV) or 'Heimdal' (Kerberos5) implementation of Kerberos. These implementations are included because they are developed outside the USA/Canada and were thus available to system owners outside those countries during the era of restrictive export controls on cryptographic code from the USA. Alternatively, the MIT implementation of Kerberos is available from the ports collection as security/krb5. Creating the Initial Database This is done on the Kerberos server only. First make sure that you do not have any old Kerberos databases around. You should change to the directory /etc/kerberosIV and check that only the following files are present: &prompt.root; cd /etc/kerberosIV &prompt.root; ls README krb.conf krb.realms If any additional files (such as principal.* or master_key) exist, then use the kdb_destroy command to destroy the old Kerberos database, or if Kerberos is not running, simply delete the extra files. You should now edit the krb.conf and krb.realms files to define your Kerberos realm. In this case the realm will be EXAMPLE.COM and the server is grunt.example.com. We edit or create the krb.conf file: &prompt.root; cat krb.conf EXAMPLE.COM EXAMPLE.COM grunt.example.com admin server CS.BERKELEY.EDU okeeffe.berkeley.edu ATHENA.MIT.EDU kerberos.mit.edu ATHENA.MIT.EDU kerberos-1.mit.edu ATHENA.MIT.EDU kerberos-2.mit.edu ATHENA.MIT.EDU kerberos-3.mit.edu LCS.MIT.EDU kerberos.lcs.mit.edu TELECOM.MIT.EDU bitsy.mit.edu ARC.NASA.GOV trident.arc.nasa.gov In this case, the other realms do not need to be there. They are here as an example of how a machine may be made aware of multiple realms. You may wish to not include them for simplicity. The first line names the realm in which this system works. The other lines contain realm/host entries. The first item on a line is a realm, and the second is a host in that realm that is acting as a key distribution center. The words admin server following a host's name means that host also provides an administrative database server. For further explanation of these terms, please consult the Kerberos manual pages. Now we have to add grunt.example.com to the EXAMPLE.COM realm and also add an entry to put all hosts in the .example.com domain in the EXAMPLE.COM realm. The krb.realms file would be updated as follows: &prompt.root; cat krb.realms grunt.example.com EXAMPLE.COM .example.com EXAMPLE.COM .berkeley.edu CS.BERKELEY.EDU .MIT.EDU ATHENA.MIT.EDU .mit.edu ATHENA.MIT.EDU Again, the other realms do not need to be there. They are here as an example of how a machine may be made aware of multiple realms. You may wish to remove them to simplify things. The first line puts the specific system into the named realm. The rest of the lines show how to default systems of a particular subdomain to a named realm. Now we are ready to create the database. This only needs to run on the Kerberos server (or Key Distribution Center). Issue the kdb_init command to do this: &prompt.root; kdb_init Realm name [default ATHENA.MIT.EDU ]: EXAMPLE.COM You will be prompted for the database Master Password. It is important that you NOT FORGET this password. Enter Kerberos master key: Now we have to save the key so that servers on the local machine can pick it up. Use the kstash command to do this. &prompt.root; kstash Enter Kerberos master key: Current Kerberos master key version is 1. Master key entered. BEWARE! This saves the encrypted master password in /etc/kerberosIV/master_key. Making It All Run Two principals need to be added to the database for each system that will be secured with Kerberos. Their names are kpasswd and rcmd These two principals are made for each system, with the instance being the name of the individual system. These daemons, kpasswd and rcmd allow other systems to change Kerberos passwords and run commands like rcp, rlogin and rsh. Now let us add these entries: &prompt.root; kdb_edit Opening database... Enter Kerberos master key: Current Kerberos master key version is 1. Master key entered. BEWARE! Previous or default values are in [brackets] , enter return to leave the same, or new value. Principal name: passwd Instance: grunt <Not found>, Create [y] ? y Principal: passwd, Instance: grunt, kdc_key_ver: 1 New Password: <---- enter RANDOM here Verifying password New Password: <---- enter RANDOM here Random password [y] ? y Principal's new key version = 1 Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ? Max ticket lifetime (*5 minutes) [ 255 ] ? Attributes [ 0 ] ? Edit O.K. Principal name: rcmd Instance: grunt <Not found>, Create [y] ? Principal: rcmd, Instance: grunt, kdc_key_ver: 1 New Password: <---- enter RANDOM here Verifying password New Password: <---- enter RANDOM here Random password [y] ? Principal's new key version = 1 Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ? Max ticket lifetime (*5 minutes) [ 255 ] ? Attributes [ 0 ] ? Edit O.K. Principal name: <---- null entry here will cause an exit Creating the Server File We now have to extract all the instances which define the services on each machine. For this we use the ext_srvtab command. This will create a file which must be copied or moved by secure means to each Kerberos client's /etc/kerberosIV directory. This file must be present on each server and client, and is crucial to the operation of Kerberos. &prompt.root; ext_srvtab grunt Enter Kerberos master key: Current Kerberos master key version is 1. Master key entered. BEWARE! Generating 'grunt-new-srvtab'.... Now, this command only generates a temporary file which must be renamed to srvtab so that all the servers can pick it up. Use the mv command to move it into place on the original system: &prompt.root; mv grunt-new-srvtab srvtab If the file is for a client system, and the network is not deemed safe, then copy the client-new-srvtab to removable media and transport it by secure physical means. Be sure to rename it to srvtab in the client's /etc/kerberosIV directory, and make sure it is mode 600: &prompt.root; mv grumble-new-srvtab srvtab &prompt.root; chmod 600 srvtab Populating the Database We now have to add some user entries into the database. First let us create an entry for the user jane. Use the kdb_edit command to do this: &prompt.root; kdb_edit Opening database... Enter Kerberos master key: Current Kerberos master key version is 1. Master key entered. BEWARE! Previous or default values are in [brackets] , enter return to leave the same, or new value. Principal name: jane Instance: <Not found>, Create [y] ? y Principal: jane, Instance: , kdc_key_ver: 1 New Password: <---- enter a secure password here Verifying password New Password: <---- re-enter the password here Principal's new key version = 1 Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ? Max ticket lifetime (*5 minutes) [ 255 ] ? Attributes [ 0 ] ? Edit O.K. Principal name: <---- null entry here will cause an exit Testing It All Out First we have to start the Kerberos daemons. NOTE that if you have correctly edited your /etc/rc.conf then this will happen automatically when you reboot. This is only necessary on the Kerberos server. Kerberos clients will automatically get what they need from the /etc/kerberosIV directory. &prompt.root; kerberos & Kerberos server starting Sleep forever on error Log file is /var/log/kerberos.log Current Kerberos master key version is 1. Master key entered. BEWARE! Current Kerberos master key version is 1 Local realm: EXAMPLE.COM &prompt.root; kadmind -n & KADM Server KADM0.0A initializing Please do not use 'kill -9' to kill this job, use a regular kill instead Current Kerberos master key version is 1. Master key entered. BEWARE! Now we can try using the kinit command to get a ticket for the id jane that we created above: &prompt.user; kinit jane MIT Project Athena (grunt.example.com) Kerberos Initialization for "jane" Password: Try listing the tokens using klist to see if we really have them: &prompt.user; klist Ticket file: /tmp/tkt245 Principal: jane@EXAMPLE.COM Issued Expires Principal Apr 30 11:23:22 Apr 30 19:23:22 krbtgt.EXAMPLE.COM@EXAMPLE.COM Now try changing the password using passwd to check if the kpasswd daemon can get authorization to the Kerberos database: &prompt.user; passwd realm EXAMPLE.COM Old password for jane: New Password for jane: Verifying password New Password for jane: Password changed. Adding <command>su</command> Privileges Kerberos allows us to give each user who needs root privileges their own separate su password. We could now add an id which is authorized to su to root. This is controlled by having an instance of root associated with a principal. Using kdb_edit we can create the entry jane.root in the Kerberos database: &prompt.root; kdb_edit Opening database... Enter Kerberos master key: Current Kerberos master key version is 1. Master key entered. BEWARE! Previous or default values are in [brackets] , enter return to leave the same, or new value. Principal name: jane Instance: root <Not found>, Create [y] ? y Principal: jane, Instance: root, kdc_key_ver: 1 New Password: <---- enter a SECURE password here Verifying password New Password: <---- re-enter the password here Principal's new key version = 1 Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ? Max ticket lifetime (*5 minutes) [ 255 ] ? 12 <--- Keep this short! Attributes [ 0 ] ? Edit O.K. Principal name: <---- null entry here will cause an exit Now try getting tokens for it to make sure it works: &prompt.root; kinit jane.root MIT Project Athena (grunt.example.com) Kerberos Initialization for "jane.root" Password: Now we need to add the user to root's .klogin file: &prompt.root; cat /root/.klogin jane.root@EXAMPLE.COM Now try doing the su: &prompt.user; su Password: and take a look at what tokens we have: &prompt.root; klist Ticket file: /tmp/tkt_root_245 Principal: jane.root@EXAMPLE.COM Issued Expires Principal May 2 20:43:12 May 3 04:43:12 krbtgt.EXAMPLE.COM@EXAMPLE.COM Using Other Commands In an earlier example, we created a principal called jane with an instance root. This was based on a user with the same name as the principal, and this is a Kerberos default; that a <principal>.<instance> of the form <username>.root will allow that <username> to su to root if the necessary entries are in the .klogin file in root's home directory: &prompt.root; cat /root/.klogin jane.root@EXAMPLE.COM Likewise, if a user has in their own home directory lines of the form: &prompt.user; cat ~/.klogin jane@EXAMPLE.COM jack@EXAMPLE.COM This allows anyone in the EXAMPLE.COM realm who has authenticated themselves to jane or jack (via kinit, see above) access to rlogin to jane's account or files on this system (grunt) via rlogin, rsh or rcp. For example, jane now logs into another system using Kerberos: &prompt.user; kinit MIT Project Athena (grunt.example.com) Password: &prompt.user; rlogin grunt Last login: Mon May 1 21:14:47 from grumble Copyright (c) 1980, 1983, 1986, 1988, 1990, 1991, 1993, 1994 The Regents of the University of California. All rights reserved. FreeBSD BUILT-19950429 (GR386) #0: Sat Apr 29 17:50:09 SAT 1995 Or Jack logs into Jane's account on the same machine (jane having set up the .klogin file as above, and the person in charge of Kerberos having set up principal jack with a null instance: &prompt.user; kinit &prompt.user; rlogin grunt -l jane MIT Project Athena (grunt.example.com) Password: Last login: Mon May 1 21:16:55 from grumble Copyright (c) 1980, 1983, 1986, 1988, 1990, 1991, 1993, 1994 The Regents of the University of California. All rights reserved. FreeBSD BUILT-19950429 (GR386) #0: Sat Apr 29 17:50:09 SAT 1995 Gary Palmer Contributed by Alex Nash Firewalls firewall security firewalls Firewalls are an area of increasing interest for people who are connected to the Internet, and are even finding applications on private networks to provide enhanced security. This section will hopefully explain what firewalls are, how to use them, and how to use the facilities provided in the FreeBSD kernel to implement them. People often think that having a firewall between your internal network and the Big Bad Internet will solve all your security problems. It may help, but a poorly setup firewall system is more of a security risk than not having one at all. A firewall can add another layer of security to your systems, but it cannot stop a really determined cracker from penetrating your internal network. If you let internal security lapse because you believe your firewall to be impenetrable, you have just made the crackers job that much easier. What Is a Firewall? There are currently two distinct types of firewalls in common use on the Internet today. The first type is more properly called a packet filtering router. This type of firewall utilizes a multi-homed machine and a set of rules to determine whether to forward or block individual packets. A multi-homed machine is simply a device with multiple network interfaces. The second type, known as a proxy server, relies on daemons to provide authentication and to forward packets, possibly on a multi-homed machine which has kernel packet forwarding disabled. Sometimes sites combine the two types of firewalls, so that only a certain machine (known as a bastion host) is allowed to send packets through a packet filtering router onto an internal network. Proxy services are run on the bastion host, which are generally more secure than normal authentication mechanisms. FreeBSD comes with a kernel packet filter (known as IPFW), which is what the rest of this section will concentrate on. Proxy servers can be built on FreeBSD from third party software, but there is such a variety of proxy servers available that it would be impossible to cover them in this section. Packet Filtering Routers A router is a machine which forwards packets between two or more networks. A packet filtering router is programmed to compare each packet to a list of rules before deciding if it should be forwarded or not. Most modern IP routing software includes packet filtering functionality that defaults to forwarding all packets. To enable the filters, you need to define a set of rules. To decide whether a packet should be passed on, the firewall looks through its set of rules for a rule which matches the contents of the packet's headers. Once a match is found, the rule action is obeyed. The rule action could be to drop the packet, to forward the packet, or even to send an ICMP message back to the originator. Only the first match counts, as the rules are searched in order. Hence, the list of rules can be referred to as a rule chain. The packet-matching criteria varies depending on the software used, but typically you can specify rules which depend on the source IP address of the packet, the destination IP address, the source port number, the destination port number (for protocols which support ports), or even the packet type (UDP, TCP, ICMP, etc). Proxy Servers Proxy servers are machines which have had the normal system daemons (telnetd, ftpd, etc) replaced with special servers. These servers are called proxy servers, as they normally only allow onward connections to be made. This enables you to run (for example) a proxy telnet server on your firewall host, and people can telnet in to your firewall from the outside, go through some authentication mechanism, and then gain access to the internal network (alternatively, proxy servers can be used for signals coming from the internal network and heading out). Proxy servers are normally more secure than normal servers, and often have a wider variety of authentication mechanisms available, including one-shot password systems so that even if someone manages to discover what password you used, they will not be able to use it to gain access to your systems as the password expires immediately after the first use. As they do not actually give users access to the host machine, it becomes a lot more difficult for someone to install backdoors around your security system. Proxy servers often have ways of restricting access further, so that only certain hosts can gain access to the servers. Most will also allow the administrator to specify which users can talk to which destination machines. Again, what facilities are available depends largely on what proxy software you choose. What Does IPFW Allow Me to Do? ipfw IPFW, the software supplied with FreeBSD, is a packet filtering and accounting system which resides in the kernel, and has a user-land control utility, &man.ipfw.8;. Together, they allow you to define and query the rules used by the kernel in its routing decisions. There are two related parts to IPFW. The firewall section performs packet filtering. There is also an IP accounting section which tracks usage of the router, based on rules similar to those used in the firewall section. This allows the administrator to monitor how much traffic the router is getting from a certain machine, or how much WWW traffic it is forwarding, for example. As a result of the way that IPFW is designed, you can use IPFW on non-router machines to perform packet filtering on incoming and outgoing connections. This is a special case of the more general use of IPFW, and the same commands and techniques should be used in this situation. Enabling IPFW on FreeBSD ipfw enabling As the main part of the IPFW system lives in the kernel, you will need to add one or more options to your kernel configuration file, depending on what facilities you want, and recompile your kernel. See "Reconfiguring your Kernel" () for more details on how to recompile your kernel. IPFW defaults to a policy of deny ip from any to any. If you do not add other rules during startup to allow access, you will lock yourself out of the server upon rebooting into a firewall-enabled kernel. We suggest that you set firewall_type=open in your /etc/rc.conf file when first enabling this feature, then refining the firewall rules in /etc/rc.firewall after you have tested that the new kernel feature works properly. To be on the safe side, you may wish to consider performing the initial firewall configuration from the local console rather than via ssh. Another option is to build a kernel using both the IPFIREWALL and IPFIREWALL_DEFAULT_TO_ACCEPT options. This will change the default rule of IPFW to allow ip from any to any and avoid the possibility of a lockout. There are currently four kernel configuration options relevant to IPFW: options IPFIREWALL Compiles into the kernel the code for packet filtering. options IPFIREWALL_VERBOSE Enables code to allow logging of packets through &man.syslogd.8;. Without this option, even if you specify that packets should be logged in the filter rules, nothing will happen. options IPFIREWALL_VERBOSE_LIMIT=10 Limits the number of packets logged through &man.syslogd.8; on a per entry basis. You may wish to use this option in hostile environments in which you want to log firewall activity, but do not want to be open to a denial of service attack via syslog flooding. When a chain entry reaches the packet limit specified, logging is turned off for that particular entry. To resume logging, you will need to reset the associated counter using the &man.ipfw.8; utility: &prompt.root; ipfw zero 4500 Where 4500 is the chain entry you wish to continue logging. options IPFIREWALL_DEFAULT_TO_ACCEPT This changes the default rule action from deny to allow. This avoids the possibility of locking yourself out if you happen to boot a kernel with IPFIREWALL support but have not configured your firewall yet. It is also very useful if you often use &man.ipfw.8; as a filter for specific problems as they arise. Use with care though, as this opens up the firewall and changes the way it works. Previous versions of FreeBSD contained an IPFIREWALL_ACCT option. This is now obsolete as the firewall code automatically includes accounting facilities. Configuring IPFW ipfw configuring The configuration of the IPFW software is done through the &man.ipfw.8; utility. The syntax for this command looks quite complicated, but it is relatively simple once you understand its structure. There are currently four different command categories used by the utility: addition/deletion, listing, flushing, and clearing. Addition/deletion is used to build the rules that control how packets are accepted, rejected, and logged. Listing is used to examine the contents of your rule set (otherwise known as the chain) and packet counters (accounting). Flushing is used to remove all entries from the chain. Clearing is used to zero out one or more accounting entries. Altering the IPFW Rules The syntax for this form of the command is: ipfw -N command index action log protocol addresses options There is one valid flag when using this form of the command: -N Resolve addresses and service names in output. The command given can be shortened to the shortest unique form. The valid commands are: add Add an entry to the firewall/accounting rule list delete Delete an entry from the firewall/accounting rule list Previous versions of IPFW used separate firewall and accounting entries. The present version provides packet accounting with each firewall entry. If an index value is supplied, it is used to place the entry at a specific point in the chain. Otherwise, the entry is placed at the end of the chain at an index 100 greater than the last chain entry (this does not include the default policy, rule 65535, deny). The log option causes matching rules to be output to the system console if the kernel was compiled with IPFIREWALL_VERBOSE. Valid actions are: reject Drop the packet, and send an ICMP host or port unreachable (as appropriate) packet to the source. allow Pass the packet on as normal. (aliases: pass, permit, and accept) deny Drop the packet. The source is not notified via an ICMP message (thus it appears that the packet never arrived at the destination). count Update packet counters but do not allow/deny the packet based on this rule. The search continues with the next chain entry. Each action will be recognized by the shortest unambiguous prefix. The protocols which can be specified are: all Matches any IP packet icmp Matches ICMP packets tcp Matches TCP packets udp Matches UDP packets The address specification is: from address/maskport to address/maskport via interface You can only specify port in conjunction with protocols which support ports (UDP and TCP). The is optional and may specify the IP address or domain name of a local IP interface, or an interface name (e.g. ed0) to match only packets coming through this interface. Interface unit numbers can be specified with an optional wildcard. For example, ppp* would match all kernel PPP interfaces. The syntax used to specify an address/mask is: address or address/mask-bits or address:mask-pattern A valid hostname may be specified in place of the IP address. is a decimal number representing how many bits in the address mask should be set. e.g. specifying 192.216.222.1/24 will create a mask which will allow any address in a class C subnet (in this case, 192.216.222) to be matched. is an IP address which will be logically AND'ed with the address given. The keyword any may be used to specify any IP address. The port numbers to be blocked are specified as: port,port,port… to specify either a single port or a list of ports, or port-port to specify a range of ports. You may also combine a single range with a list, but the range must always be specified first. The options available are: frag Matches if the packet is not the first fragment of the datagram. in Matches if the packet is on the way in. out Matches if the packet is on the way out. ipoptions spec Matches if the IP header contains the comma separated list of options specified in spec. The supported IP options are: ssrr (strict source route), lsrr (loose source route), rr (record packet route), and ts (time stamp). The absence of a particular option may be specified with a leading !. established Matches if the packet is part of an already established TCP connection (i.e. it has the RST or ACK bits set). You can optimize the performance of the firewall by placing established rules early in the chain. setup Matches if the packet is an attempt to establish a TCP connection (the SYN bit is set but the ACK bit is not). tcpflags flags Matches if the TCP header contains the comma separated list of flags. The supported flags are fin, syn, rst, psh, ack, and urg. The absence of a particular flag may be indicated by a leading !. icmptypes types Matches if the ICMP type is present in the list types. The list may be specified as any combination of ranges and/or individual types separated by commas. Commonly used ICMP types are: 0 echo reply (ping reply), 3 destination unreachable, 5 redirect, 8 echo request (ping request), and 11 time exceeded (used to indicate TTL expiration as with &man.traceroute.8;). Listing the IPFW Rules The syntax for this form of the command is: ipfw -a -c -d -e -t -N -S list There are seven valid flags when using this form of the command: -a While listing, show counter values. This option is the only way to see accounting counters. -c List rules in compact form. -d Show dynamic rules in addition to static rules. -e If was specified, also show expired dynamic rules. -t Display the last match times for each chain entry. The time listing is incompatible with the input syntax used by the &man.ipfw.8; utility. -N Attempt to resolve given addresses and service names. -S Show the set each rule belongs to. If this flag is not specified, disabled rules will not be listed. Flushing the IPFW Rules The syntax for flushing the chain is: ipfw flush This causes all entries in the firewall chain to be removed except the fixed default policy enforced by the kernel (index 65535). Use caution when flushing rules; the default deny policy will leave your system cut off from the network until allow entries are added to the chain. Clearing the IPFW Packet Counters The syntax for clearing one or more packet counters is: ipfw zero index When used without an index argument, all packet counters are cleared. If an index is supplied, the clearing operation only affects a specific chain entry. Example Commands for <application>ipfw</application> This command will deny all packets from the host evil.crackers.org to the telnet port of the host nice.people.org: &prompt.root; ipfw add deny tcp from evil.crackers.org to nice.people.org 23 The next example denies and logs any TCP traffic from the entire crackers.org network (a class C) to the nice.people.org machine (any port). &prompt.root; ipfw add deny log tcp from evil.crackers.org/24 to nice.people.org If you do not want people sending X sessions to your internal network (a subnet of a class C), the following command will do the necessary filtering: &prompt.root; ipfw add deny tcp from any to my.org/28 6000 setup To see the accounting records: &prompt.root; ipfw -a list or in the short form &prompt.root; ipfw -a l You can also see the last time a chain entry was matched with: &prompt.root; ipfw -at l Building a Packet Filtering Firewall The following suggestions are just that: suggestions. The requirements of each firewall are different and we cannot tell you how to build a firewall to meet your particular requirements. When initially setting up your firewall, unless you have a test bench setup where you can configure your firewall host in a controlled environment, it is strongly recommend you use the logging version of the commands and enable logging in the kernel. This will allow you to quickly identify problem areas and cure them without too much disruption. Even after the initial setup phase is complete, I recommend using the logging for `deny' as it allows tracing of possible attacks and also modification of the firewall rules if your requirements alter. If you use the logging versions of the accept command, be aware that it can generate large amounts of log data. One log entry will be generated for every packet that passes through the firewall, so large FTP/http transfers, etc, will really slow the system down. It also increases the latencies on those packets as it requires more work to be done by the kernel before the packet can be passed on. syslogd will also start using up a lot more processor time as it logs all the extra data to disk, and it could quite easily fill the partition /var/log is located on. You should enable your firewall from /etc/rc.conf.local or /etc/rc.conf. The associated manual page explains which knobs to fiddle and lists some preset firewall configurations. If you do not use a preset configuration, ipfw list will output the current ruleset into a file that you can pass to rc.conf. If you do not use /etc/rc.conf.local or /etc/rc.conf to enable your firewall, it is important to make sure your firewall is enabled before any IP interfaces are configured. The next problem is what your firewall should actually do! This is largely dependent on what access to your network you want to allow from the outside, and how much access to the outside world you want to allow from the inside. Some general rules are: Block all incoming access to ports below 1024 for TCP. This is where most of the security sensitive services are, like finger, SMTP (mail) and telnet. Block all incoming UDP traffic. There are very few useful services that travel over UDP, and what useful traffic there is, is normally a security threat (e.g. Suns RPC and NFS protocols). This has its disadvantages also, since UDP is a connectionless protocol, denying incoming UDP traffic also blocks the replies to outgoing UDP traffic. This can cause a problem for people (on the inside) using external archie (prospero) servers. If you want to allow access to archie, you will have to allow packets coming from ports 191 and 1525 to any internal UDP port through the firewall. ntp is another service you may consider allowing through, which comes from port 123. Block traffic to port 6000 from the outside. Port 6000 is the port used for access to X11 servers, and can be a security threat (especially if people are in the habit of doing xhost + on their workstations). X11 can actually use a range of ports starting at 6000, the upper limit being how many X displays you can run on the machine. The upper limit as defined by RFC 1700 (Assigned Numbers) is 6063. Check what ports any internal servers use (e.g. SQL servers, etc). It is probably a good idea to block those as well, as they normally fall outside the 1-1024 range specified above. Another checklist for firewall configuration is available from CERT at As stated above, these are only guidelines. You will have to decide what filter rules you want to use on your firewall yourself. We cannot accept ANY responsibility if someone breaks into your network, even if you follow the advice given above. IPFW Overhead and Optimization Many people want to know how much overhead IPFW adds to a system. The answer to this depends mostly on your rule set and processor speed. For most applications dealing with Ethernet and small rule sets, the answer is negligible. For those of you that need actual measurements to satisfy your curiosity, read on. The following measurements were made using 2.2.5-STABLE on a 486-66. (While IPFW has changed slightly in later releases of FreeBSD, it still performs with similar speed.) IPFW was modified to measure the time spent within the ip_fw_chk routine, displaying the results to the console every 1000 packets. Two rule sets, each with 1000 rules, were tested. The first set was designed to demonstrate a worst case scenario by repeating the rule: &prompt.root; ipfw add deny tcp from any to any 55555 This demonstrates a worst case scenario by causing most of IPFW's packet check routine to be executed before finally deciding that the packet does not match the rule (by virtue of the port number). Following the 999th iteration of this rule was an allow ip from any to any. The second set of rules were designed to abort the rule check quickly: &prompt.root; ipfw add deny ip from 1.2.3.4 to 1.2.3.4 The non-matching source IP address for the above rule causes these rules to be skipped very quickly. As before, the 1000th rule was an allow ip from any to any. The per-packet processing overhead in the former case was approximately 2.703 ms/packet, or roughly 2.7 microseconds per rule. Thus the theoretical packet processing limit with these rules is around 370 packets per second. Assuming 10 Mbps Ethernet and a ~1500 byte packet size, we would only be able to achieve 55.5% bandwidth utilization. For the latter case each packet was processed in approximately 1.172 ms, or roughly 1.2 microseconds per rule. The theoretical packet processing limit here would be about 853 packets per second, which could consume 10 Mbps Ethernet bandwidth. The excessive number of rules tested and the nature of those rules do not provide a real-world scenario -- they were used only to generate the timing information presented here. Here are a few things to keep in mind when building an efficient rule set: Place an established rule early on to handle the majority of TCP traffic. Do not put any allow tcp statements before this rule. Place heavily triggered rules earlier in the rule set than those rarely used (without changing the permissiveness of the firewall, of course). You can see which rules are used most often by examining the packet counting statistics with ipfw -a l. OpenSSL security OpenSSL OpenSSL As of FreeBSD 4.0, the OpenSSL toolkit is a part of the base system. OpenSSL provides a general-purpose cryptography library, as well as the Secure Sockets Layer v2/v3 (SSLv2/SSLv3) and Transport Layer Security v1 (TLSv1) network security protocols. However, one of the algorithms (specifically IDEA) included in OpenSSL is protected by patents in the USA and elsewhere, and is not available for unrestricted use. IDEA is included in the OpenSSL sources in FreeBSD, but it is not built by default. If you wish to use it, and you comply with the license terms, enable the MAKE_IDEA switch in /etc/make.conf and rebuild your sources using make world. Today, the RSA algorithm is free for use in USA and other countries. In the past it was protected by a patent. OpenSSL install Source Code Installations OpenSSL is part of the src-crypto and src-secure cvsup collections. See the Obtaining FreeBSD section for more information about obtaining and updating FreeBSD source code. Yoshinobu Inoue Contributed by IPsec IPsec security IPsec Terminating Characters Throughout examples in this section, and other sections, you will notice that there is a ^D at the end of some examples. This means to hold down the Control key and hit the D key. Another commonly used character is ^C, which respectively means to hold down Control and press C. For other HOWTOs detailing IPsec implementation in FreeBSD, take a look at and . The IPsec mechanism provides secure communication for IP layer and socket layer communication. This section should explain how to use them. For implementation details, please refer to The Developers' Handbook. The current IPsec implementation supports both transport mode and tunnel mode. However, tunnel mode comes with some restrictions. has more comprehensive examples. Please be aware that in order to use this functionality, you must have the following options compiled into your kernel: options IPSEC #IP security options IPSEC_ESP #IP security (crypto; define w/IPSEC) Transport Mode Example with IPv4 Let us setup security association to deploy a secure channel between HOST A (10.2.3.4) and HOST B (10.6.7.8). Here we show a little complicated example. From HOST A to HOST B, only old AH is used. From HOST B to HOST A, new AH and new ESP are combined. Now we should choose an algorithm to be used corresponding to AH/new AH/ESP/ new ESP. Please refer to the &man.setkey.8; man page to know algorithm names. Our choice is MD5 for AH, new-HMAC-SHA1 for new AH, and new-DES-expIV with 8 byte IV for new ESP. Key length highly depends on each algorithm. For example, key length must be equal to 16 bytes for MD5, 20 for new-HMAC-SHA1, and 8 for new-DES-expIV. Now we choose MYSECRETMYSECRET, KAMEKAMEKAMEKAMEKAME, PASSWORD, respectively. OK, let us assign SPI (Security Parameter Index) for each protocol. Please note that we need 3 SPIs for this secure channel since three security headers are produced (one for from HOST A to HOST B, two for from HOST B to HOST A). Please also note that SPI MUST be greater than or equal to 256. We choose, 1000, 2000, and 3000, respectively. (1) HOST A ------> HOST B (1)PROTO=AH ALG=MD5(RFC1826) KEY=MYSECRETMYSECRET SPI=1000 (2.1) HOST A <------ HOST B <------ (2.2) (2.1) PROTO=AH ALG=new-HMAC-SHA1(new AH) KEY=KAMEKAMEKAMEKAMEKAME SPI=2000 (2.2) PROTO=ESP ALG=new-DES-expIV(new ESP) IV length = 8 KEY=PASSWORD SPI=3000 Now, let us setup security association. Execute &man.setkey.8; on both HOST A and B: &prompt.root; setkey -c add 10.2.3.4 10.6.7.8 ah-old 1000 -m transport -A keyed-md5 "MYSECRETMYSECRET" ; add 10.6.7.8 10.2.3.4 ah 2000 -m transport -A hmac-sha1 "KAMEKAMEKAMEKAMEKAME" ; add 10.6.7.8 10.2.3.4 esp 3000 -m transport -E des-cbc "PASSWORD" ; ^D Actually, IPsec communication does not process until security policy entries are defined. In this case, you must setup each host. At A: &prompt.root; setkey -c spdadd 10.2.3.4 10.6.7.8 any -P out ipsec ah/transport/10.2.3.4-10.6.7.8/require ; ^D At B: &prompt.root; setkey -c spdadd 10.6.7.8 10.2.3.4 any -P out ipsec esp/transport/10.6.7.8-10.2.3.4/require ; spdadd 10.6.7.8 10.2.3.4 any -P out ipsec ah/transport/10.6.7.8-10.2.3.4/require ; ^D HOST A --------------------------------------> HOST E 10.2.3.4 10.6.7.8 | | ========== old AH keyed-md5 ==========> <========= new AH hmac-sha1 =========== <========= new ESP des-cbc ============ Transport Mode Example with IPv6 Another example using IPv6. ESP transport mode is recommended for TCP port number 110 between Host-A and Host-B. ============ ESP ============ | | Host-A Host-B fec0::10 -------------------- fec0::11 Encryption algorithm is blowfish-cbc whose key is kamekame, and authentication algorithm is hmac-sha1 whose key is this is the test key. Configuration at Host-A: &prompt.root; setkey -c <<EOF spdadd fec0::10[any] fec0::11[110] tcp -P out ipsec esp/transport/fec0::10-fec0::11/use ; spdadd fec0::11[110] fec0::10[any] tcp -P in ipsec esp/transport/fec0::11-fec0::10/use ; add fec0::10 fec0::11 esp 0x10001 -m transport -E blowfish-cbc "kamekame" -A hmac-sha1 "this is the test key" ; add fec0::11 fec0::10 esp 0x10002 -m transport -E blowfish-cbc "kamekame" -A hmac-sha1 "this is the test key" ; EOF and at Host-B: &prompt.root; setkey -c <<EOF spdadd fec0::11[110] fec0::10[any] tcp -P out ipsec esp/transport/fec0::11-fec0::10/use ; spdadd fec0::10[any] fec0::11[110] tcp -P in ipsec esp/transport/fec0::10-fec0::11/use ; add fec0::10 fec0::11 esp 0x10001 -m transport -E blowfish-cbc "kamekame" -A hmac-sha1 "this is the test key" ; add fec0::11 fec0::10 esp 0x10002 -m transport -E blowfish-cbc "kamekame" -A hmac-sha1 "this is the test key" ; EOF Note the direction of SP. Tunnel Mode Example with IPv4 Tunnel mode between two security gateways Security protocol is old AH tunnel mode, i.e. specified by RFC1826, with keyed-md5 whose key is this is the test as authentication algorithm. ======= AH ======= | | Network-A Gateway-A Gateway-B Network-B 10.0.1.0/24 ---- 172.16.0.1 ----- 172.16.0.2 ---- 10.0.2.0/24 Configuration at Gateway-A: &prompt.root; setkey -c <<EOF spdadd 10.0.1.0/24 10.0.2.0/24 any -P out ipsec ah/tunnel/172.16.0.1-172.16.0.2/require ; spdadd 10.0.2.0/24 10.0.1.0/24 any -P in ipsec ah/tunnel/172.16.0.2-172.16.0.1/require ; add 172.16.0.1 172.16.0.2 ah-old 0x10003 -m any -A keyed-md5 "this is the test" ; add 172.16.0.2 172.16.0.1 ah-old 0x10004 -m any -A keyed-md5 "this is the test" ; EOF If the port number field is omitted such as above then [any] is employed. -m specifies the mode of SA to be used. -m any means wild-card of mode of security protocol. You can use this SA for both tunnel and transport mode. and at Gateway-B: &prompt.root; setkey -c <<EOF spdadd 10.0.2.0/24 10.0.1.0/24 any -P out ipsec ah/tunnel/172.16.0.2-172.16.0.1/require ; spdadd 10.0.1.0/24 10.0.2.0/24 any -P in ipsec ah/tunnel/172.16.0.1-172.16.0.2/require ; add 172.16.0.1 172.16.0.2 ah-old 0x10003 -m any -A keyed-md5 "this is the test" ; add 172.16.0.2 172.16.0.1 ah-old 0x10004 -m any -A keyed-md5 "this is the test" ; EOF Making SA bundle between two security gateways AH transport mode and ESP tunnel mode is required between Gateway-A and Gateway-B. In this case, ESP tunnel mode is applied first, and AH transport mode is next. ========== AH ========= | ======= ESP ===== | | | | | Network-A Gateway-A Gateway-B Network-B fec0:0:0:1::/64 --- fec0:0:0:1::1 ---- fec0:0:0:2::1 --- fec0:0:0:2::/64 Tunnel Mode Example with IPv6 Encryption algorithm is 3des-cbc, and authentication algorithm for ESP is hmac-sha1. Authentication algorithm for AH is hmac-md5. Configuration at Gateway-A: &prompt.root; setkey -c <<EOF spdadd fec0:0:0:1::/64 fec0:0:0:2::/64 any -P out ipsec esp/tunnel/fec0:0:0:1::1-fec0:0:0:2::1/require ah/transport/fec0:0:0:1::1-fec0:0:0:2::1/require ; spdadd fec0:0:0:2::/64 fec0:0:0:1::/64 any -P in ipsec esp/tunnel/fec0:0:0:2::1-fec0:0:0:1::1/require ah/transport/fec0:0:0:2::1-fec0:0:0:1::1/require ; add fec0:0:0:1::1 fec0:0:0:2::1 esp 0x10001 -m tunnel -E 3des-cbc "kamekame12341234kame1234" -A hmac-sha1 "this is the test key" ; add fec0:0:0:1::1 fec0:0:0:2::1 ah 0x10001 -m transport -A hmac-md5 "this is the test" ; add fec0:0:0:2::1 fec0:0:0:1::1 esp 0x10001 -m tunnel -E 3des-cbc "kamekame12341234kame1234" -A hmac-sha1 "this is the test key" ; add fec0:0:0:2::1 fec0:0:0:1::1 ah 0x10001 -m transport -A hmac-md5 "this is the test" ; EOF Making SAs with the different end ESP tunnel mode is required between Host-A and Gateway-A. Encryption algorithm is cast128-cbc, and authentication algorithm for ESP is hmac-sha1. ESP transport mode is recommended between Host-A and Host-B. Encryption algorithm is rc5-cbc, and authentication algorithm for ESP is hmac-md5. ================== ESP ================= | ======= ESP ======= | | | | | Host-A Gateway-A Host-B fec0:0:0:1::1 ---- fec0:0:0:2::1 ---- fec0:0:0:2::2 Configuration at Host-A: &prompt.root; setkey -c <<EOF spdadd fec0:0:0:1::1[any] fec0:0:0:2::2[80] tcp -P out ipsec esp/transport/fec0:0:0:1::1-fec0:0:0:2::2/use esp/tunnel/fec0:0:0:1::1-fec0:0:0:2::1/require ; spdadd fec0:0:0:2::1[80] fec0:0:0:1::1[any] tcp -P in ipsec esp/transport/fec0:0:0:2::2-fec0:0:0:l::1/use esp/tunnel/fec0:0:0:2::1-fec0:0:0:1::1/require ; add fec0:0:0:1::1 fec0:0:0:2::2 esp 0x10001 -m transport -E cast128-cbc "12341234" -A hmac-sha1 "this is the test key" ; add fec0:0:0:1::1 fec0:0:0:2::1 esp 0x10002 -E rc5-cbc "kamekame" -A hmac-md5 "this is the test" ; add fec0:0:0:2::2 fec0:0:0:1::1 esp 0x10003 -m transport -E cast128-cbc "12341234" -A hmac-sha1 "this is the test key" ; add fec0:0:0:2::1 fec0:0:0:1::1 esp 0x10004 -E rc5-cbc "kamekame" -A hmac-md5 "this is the test" ; EOF Chern Lee Contributed by OpenSSH OpenSSH security OpenSSH Secure shell is a set of network connectivity tools used to access remote machines securely. It can be used as a direct replacement for rlogin, rsh, rcp, and telnet. Additionally, any other TCP/IP connections can be tunneled/forwarded securely through ssh. ssh encrypts all traffic to effectively eliminate eavesdropping, connection hijacking, and other network-level attacks. OpenSSH is maintained by the OpenBSD project, and is based upon SSH v1.2.12 with all the recent bug fixes and updates. It is compatible with both SSH protocols 1 and 2. OpenSSH has been in the base system since FreeBSD 4.0. Advantages of Using OpenSSH Normally, when using &man.telnet.1; or &man.rlogin.1;, data is sent over the network in an clear, un-encrypted form. Network sniffers anywhere in between the client and server can steal your user/password information or data transferred in your session. OpenSSH offers a variety of authentication and encryption methods to prevent this from happening. Enabling sshd OpenSSH enabling Be sure to make the following additions to your rc.conf file: sshd_enable="YES" This will load the ssh daemon the next time your system initializes. Alternatively, you can simply run the sshd daemon. SSH Client OpenSSH client The &man.ssh.1; utility works similarly to &man.rlogin.1;. &prompt.root; ssh user@example.com Host key not found from the list of known hosts. Are you sure you want to continue connecting (yes/no)? yes Host 'example.com' added to the list of known hosts. user@example.com's password: ******* The login will continue just as it would have if a session was created using rlogin or telnet. SSH utilizes a key fingerprint system for verifying the authenticity of the server when the client connects. The user is prompted to enter yes only when connecting for the first time. Future attempts to login are all verified against the saved fingerprint key. The SSH client will alert you if the saved fingerprint differs from the received fingerprint on future login attempts. The fingerprints are saved in ~/.ssh/known_hosts, or ~/.ssh/known_hosts2 for SSH v2 fingerprints. By default, OpenSSH servers are configured to accept both SSH v1 and SSH v2 connections. The client, however, can choose between the two. Version 2 is known to be more robust and secure than its predecessor. ssh can be forced to use either protocol by passing it the or argument for v1 and v2, respectively. Secure Copy OpenSSH secure copy scp The scp command works similarly to rcp; it copies a file to or from a remote machine, except in a secure fashion. &prompt.root; scp user@example.com:/COPYRIGHT COPYRIGHT user@example.com's password: ******* COPYRIGHT 100% |*****************************| 4735 00:00 &prompt.root; Since the fingerprint was already saved for this host in the previous example, it is verified when using scp here. The arguments passed to scp are similar to cp, with the file or files in the first argument, and the destination in the second. Since the file is fetched over the network, through SSH, one or more of the file arguments takes on the form . Configuration OpenSSH configuration The system-wide configuration files for both the OpenSSH daemon and client reside within the /etc/ssh directory. ssh_config configures the client settings, while sshd_config configures the daemon. Additionally, the (/usr/sbin/sshd by default), and rc.conf options can provide more levels of configuration. ssh-keygen Instead of using passwords, &man.ssh-keygen.1; can be used to generate RSA keys to authenticate a user. &prompt.user; ssh-keygen Initializing random number generator... Generating p: .++ (distance 66) Generating q: ..............................++ (distance 498) Computing the keys... Key generation complete. Enter file in which to save the key (/home/user/.ssh/identity): Enter passphrase: Enter the same passphrase again: Your identification has been saved in /home/user/.ssh/identity. ... &man.ssh-keygen.1; will create a public and private key pair for use in authentication. The private key is stored in ~/.ssh/identity, whereas the public key is stored in ~/.ssh/identity.pub. The public key must be placed in ~/.ssh/authorized_keys of the remote machine in order for the setup to work. This will allow connection to the remote machine based upon RSA authentication instead of passwords. If a passphrase is used in &man.ssh-keygen.1;, the user will be prompted for a password each time in order to use the private key. A SSH v2 DSA key can be created for the same purpose by using the ssh-keygen -d command (or ssh-keygen -t dsa for FreeBSD &os.current;). This will create a public/private DSA key for use in SSH v2 sessions only. The public key is stored in ~/.ssh/id_dsa.pub, while the private key is in ~/.ssh/id_dsa. DSA public keys are placed in ~/.ssh/authorized_keys2 on the remote machine. &man.ssh-agent.1; and &man.ssh-add.1; are utilities used in managing multiple passworded private keys. SSH Tunneling OpenSSH tunneling OpenSSH has the ability to create a tunnel to encapsulate another protocol in an encrypted session. The following command tells &man.ssh.1; to create a tunnel for telnet. &prompt.user; ssh -2 -N -f -L 5023:localhost:23 user@foo.example.com &prompt.user; The ssh command is used with the following options: Forces ssh to use version 2 of the protocol. (Do not use if you are working with older ssh servers) Indicates no command, or tunnel only. If omitted, ssh would initiate a normal session. Forces ssh to run in the background. Indicates a local tunnel in localport:remotehost:remoteport fashion. The remote SSH server. An SSH tunnel works by creating a listen socket on localhost on the specified port. It then forwards any connection received on the local host/port via the SSH connection to the specified remote host and port. In the example, port 5023 on localhost is being forwarded to port 23 on localhost of the remote machine. Since 23 is telnet, this would create a secure telnet session through an SSH tunnel. This can be used to wrap any number of insecure TCP protocols such as SMTP, POP3, FTP, etc. Using SSH to create a secure tunnel for SMTP &prompt.user; ssh -2 -N -f -L 5025:localhost:25 user@mailserver.example.com user@mailserver.example.com's password: ***** &prompt.user; telnet localhost 5025 Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. 220 mailserver.example.com ESMTP This can be used in conjunction with an &man.ssh-keygen.1; and additional user accounts to create a more seamless/hassle-free SSH tunneling environment. Keys can be used in place of typing a password, and the tunnels can be run as a separate user. Practical SSH Tunneling Examples Secure Access of a POP3 server At work, there is an SSH server that accepts connections from the outside. On the same office network resides a mail server running a POP3 server. The network, or network path between your home and office may or may not be completely trustable. Because of this, you need to check your e-mail in a secure manner. The solution is to create an SSH connection to your office's SSH server, and tunnel through to the mail server. &prompt.user; ssh -2 -N -f -L 2110:mail.example.com:110 user@ssh-server.example.com user@ssh-server.example.com's password: ****** When the tunnel is up and running, you can point your mail client to send POP3 requests to localhost port 2110. A connection here will be forwarded securely across the tunnel to mail.example.com. Bypassing a Draconian Firewall Some network administrators impose extremely Draconian firewall rules, filtering not only incoming connections, but outgoing connections. You may be only given access to contact remote machines on ports 22 and 80 for SSH and web surfing. You may wish to access another (perhaps non-work related) service, such as an Ogg Vorbis server to stream music. If this Ogg Vorbis server is streaming on some other port than 22 or 80, you will not be able to access it. The solution is to create an SSH connection to a machine outside of your network's firewall, and use it to tunnel to the Ogg Vorbis server. &prompt.user; ssh -2 -N -f -L 8888:music.example.com:8000 user@unfirewalled-system.example.org user@unfirewalled-system.example.org's password: ******* Your streaming client can now be pointed to localhost port 8888, which will be forwarded over to music.example.com port 8000, successfully evading the firewall. Further Reading OpenSSH &man.ssh.1; &man.scp.1; &man.ssh-keygen.1; &man.ssh-agent.1; &man.ssh-add.1; &man.sshd.8; &man.sftp-server.8; Robert Watson Sponsored by DARPA and Network Associates Laboratories. Contributed by MAC Mandatory Access Control (MAC) FreeBSD 5.0 includes a new kernel security framework, the TrustedBSD MAC Framework. The MAC Framework permits compile-time, boot-time, and run-time extension of the kernel access control policy, and can be used to load support for Mandatory Access Control (MAC), and custom security modules such as hardening modules. The MAC Framework is currently considered to be an experimental feature, and should not yet be used in production environments without careful consideration. It is anticipated that the MAC Framework will be appropriate for more widespread production use by FreeBSD 5.2. When configured into a kernel, the MAC Framework permits security modules to augment the existing kernel access control model, restricting access to system services and objects. For example, the &man.mac.bsdextended.4; module augments file system access control, permitting administrators to provide a firewall-like ruleset constraining access to file system objects based on user ids and group membership. Some modules require little or no configuration, such as &man.mac.seeotheruids.4, whereas others perform ubiquitous object labeling, such as &man.mac.biba.4; and &man.mac.mls.4;, and require extensive configuration. To enable the MAC Framework in your system kernel, you must add the following entry to your kernel configuration: options MAC Security policy modules shipped with the base system may be loaded using &man.kldload.8; or in the boot &man.loader.8; They may also be compiled directly into the kernel using the following options, if the use of modules is not desired. Different MAC policies may be configured in different ways; frequently, MAC policy modules export configuration parameters using the &man.sysctl.8; MIB using the security.mac namespace. Policies relying on file system or other labels may require a configuration step that involves assigning initial labels to system objects or creating a policy configuration file. For information on how to configure and use each policy module, see its man page. A variety of tools are available to configure the MAC Framework and labels maintained by various policies. Extensions have been made to the login and credential management mechanisms (&man.setusercontext.3;) to support initial user labeling using &man.login.conf.5;. In addition, modifications have been made to &man.su.1;, &man.ps.1;, &man.ls.1;, and &man.ifconfig.8; to inspect and set labels on processes, files, and interfaces. In addition, several new tools have been added to manage labels on objects, including &man.getfmac.8;, &man.setfmac.8;, and &man.setfsmac.8; to manage labels on files, and &man.getpmac.8; and &man.setpmac.8;. What follows is a list of policy modules shipped with FreeBSD 5.0. Biba Integrity Policy (mac_biba) Biba Integrity Policy Vendor: TrustedBSD Project Module name: mac_biba.ko Kernel option: MAC_BIBA TCB The Biba Integrity Policy (&man.mac.biba.4;) provides for hierarchical and non-hierarchical labeling of all system objects with integrity data, and the strict enforcement of an information flow policy to prevent corruption of high integrity subjects and data by low-integrity subjects. Integrity is enforced by preventing high integrity subjects (generally processes) from reading low integrity objects (often files), and preventing low integrity subjects from writing to high integrity objects. This security policy is frequently used in commercial trusted systems to provide strong protection for the Trusted Code Base (TCB). Because it provides ubiquitous labeling, the Biba integrity policy must be compiled into the kernel or loaded at boot. File System Firewall Policy (mac_bsdextended) File System Firewall Policy Vendor: TrustedBSD Project Module name: mac_bsdextended.ko Kernel option: MAC_BSDEXTENDED The File System Firewall Policy (&man.mac.bsdextended.4;) provides an extension to the BSD file system permission model, permitting the administrator to define a set of firewall-like rules for limiting access to file system objects owned by other users and groups. Managed using &man.ugidfw.8;, rules may limit access to files and directories based on the uid and gids of the process attempting the access, and the owner and group of the target of the access attempt. All rules are restrictive, so they may be placed in any order. This policy requires no prior configuration or labeling, and may be appropriate in multi-user environments where mandatory limits on inter-user data exchange are required. Caution should be exercised in limiting access to files owned by the super-user or other system user ids, as many useful programs and directories are owned by these users. As with a network firewall, improper application of file system firewall rules may render the system unusable. New tools to manage the rule set may be easily written using the &man.libugidfw.3; library. Interface Silencing Policy (mac_ifoff) Interface Silencing Policy Vendor: TrustedBSD Project Module name: mac_ifoff.ko Kernel option: MAC_IFOFF The interface silencing policy (&man.mac.ifoff.4;) prohibits the use of network interfaces during the boot until explicitly enabled, preventing spurious stack output stack response to incoming packets. This is appropriate for use in environments where the monitoring of packets is required, but no traffic may be generated. Low-Watermark Mandatory Access Control (LOMAC) (mac_lomac) MAC Low-Watermark LOMAC Vendor: Network Associates Laboratories Module name: mac_lomac.ko Kernel option: MAC_LOMAC Similar to the Biba Integrity Policy, the LOMAC policy (&man.mac.lomac.4;) relies on the ubiquitous labeling of all system objects with integrity labels. Unlike Biba, LOMAC permits high integrity subjects to read from low integrity objects, but then downgrades the label on the subject to prevent future writes to high integrity objects. This policy may provide for greater compatibility, as well as require less initial configuration than Biba. However, as with Biba, it ubiquitously labels objects and must therefore be compiled into the kernel or loaded at boot. Multi-Level Security Policy (MLS) (mac_mls) Multi-Level Security Policy MAC Multi-Level Vendor: TrustedBSD Project Module name: mac_mls.ko Kernel option: MAC_MLS Multi-Level Security (MLS) (&man.mac.mls.4;) provides for hierarchical and non-hierarchical labeling of all system objects with sensitivity data, and the strict enforcement of an information flow policy to prevent the leakage of confidential data to untrusted parties. The logical conjugate of the Biba Integrity Policy, MLS is frequently shipped in commercial trusted operating systems to protect data secrecy in multi-user environments. Hierarchal labels provide support for the notion of clearances and classifications in traditional parlance; non-hierarchical labels provide support for need-to-know. As with Biba, ubiquitous labeling of objects occurs, and it must therefore be compiled into the kernel or loaded at boot. As with Biba, extensive initial configuration may be required. MAC Stub Policy (mac_none) MAC Stub Policy Vendor: TrustedBSD Project Module name: mac_none.ko Kernel option: MAC_NONE The None policy (&man.mac.none.4;) provides a stub sample policy for developers, implementing all entry points, but not changing the system access control policy. Running this on a production system would not be highly beneficial. Process Partition Policy (mac_partition) Process Partition Policy Vendor: TrustedBSD Project Module name: mac_partition.ko Kernel option: MAC_PARTITION The Partition policy (&man.mac.partition.4;) provides for a simple process visibility limitation, assigning labels to processes identifying what numeric system partition they are present in. If none, all other processes are visible using standard monitoring tools; if a partition identifier is present, then only other processes in the same partition are visible. This policy may be compiled into the kernel, loaded at boot, or loaded at run-time. See Other Uids Policy (mac_seeotheruids) See Other Uids Policy Vendor: TrustedBSD Project Module name: mac_seeotheruids.ko Kernel option: MAC_SEEOTHERUIDS The See Other Uids policy (&man.mac.seeotheruids.4;) implements a similar process visibility model to mac_partition, except that it relies on process credentials to control visibility of processes, rather than partition labels. This policy may be configured to exempt certain users and groups, including permitting system operators to view all processes without special privilege. This policy may be compiled into the kernel, loaded at boot, or loaded at run-time. MAC Framework Test Policy (mac_test) MAC Framework Test Policy Vendor: TrustedBSD Project Module name: mac_test.ko Kernel option: MAC_TEST The Test policy (&man.mac.test.4;) provides a regression test environment for the MAC Framework, and will cause a fail-stop in the event that internal MAC Framework assertions about proper data labeling fail. This module can be used to detect failures to properly label system objects in the kernel implementation. This policy may be compiled into the kernel, loaded at boot, or loaded at run-time. - + + + + + + Tom + Rhodes + Contributed by + + + + + ACL + + File System Access Control Lists + + In conjunction with file system enhancements like snapshots, FreeBSD 5.0 + and later offers the security of File System Access Control Lists + (ACLs). + + Access Control Lists extend the standard UNIX + permission model in a highly compatible (POSIX.1e) way. This feature + permits an administrator to make use of and take advantage of a + more sophisticated security model. + + To enable ACL support for UFS + file systems, the following: + + options UFS_ACL + + must be compiled into the kernel. If this option has + not been compiled in, a warning message will be displayed + when attempting to mount a file system supporting ACLs. + This option is included in the GENERIC kernel. + ACLs rely on extended attributes being enabled on + the file system. Extended attributes are natively supported in the next generation + UNIX file system, UFS2. + + A higher level of administrative overhead is required to + configure extended attributes on UFS1 than on + UFS2. The performance of extended attributes + on UFS2 is also substantially higher. As a + result, UFS2 is generally recommended in preference + to UFS1 for use with access control lists. + + ACLs are enabled by the mount-time administrative + flag, , which may be added to /etc/fstab. + The mount-time flag can also be automatically set in a persistent manner using + &man.tunefs.8; to modify a superblock ACLs flag in the + file system header. In general, it is preferred to use the superblock flag + for several reasons: + + + + The mount-time ACLs flag cannot be changed by a + remount (&man.mount.8; ), only by means of a complete + &man.umount.8; and fresh &man.mount.8;. This means that + ACLs cannot be enabled on the root file system after boot. + It also means that you cannot change the disposition of a file system once + it is in use. + + + + Setting the superblock flag will cause the file system to always be + mounted with ACLs enabled even if there is not an + fstab entry or if the devices re-order. This prevents + accidental mounting of the file system without ACLs + enabled, which can result in ACLs being improperly enforced, + and hence security problems. + + + + We may change the ACLs behavior to allow the flag to + be enabled without a complete fresh &man.mount.8;, but we consider it desirable to + discourage accidental mounting without ACLs enabled, because you + can shoot your feet quite nastily if you enable ACLs, then disable + them, then re-enable them without flushing the extended attributes. In general, once + you have enabled ACLs on a file system, they should not be disabled, + as the resulting file protections may not be compatible with those intended by the + users of the system, and re-enabling ACLs may re-attach the previous + ACLs to files that have since had their permissions changed, + resulting in other unpredictable behavior. + + File systems with ACLs enabled will show a + + (plus) sign in their permission settings when viewed. For example: + + drwx------ 2 robert robert 512 Dec 27 11:54 private +drwxrwx---+ 2 robert robert 512 Dec 23 10:57 directory1 +drwxrwx---+ 2 robert robert 512 Dec 22 10:20 directory2 +drwxrwx---+ 2 robert robert 512 Dec 27 11:57 directory3 +drwxr-xr-x 2 robert robert 512 Nov 10 11:54 public_html + + Here we see that the directory1, + directory2, and directory3 + directories are all taking advantage of ACLs. The + public_html directory is not. +